DESCRIPTION LUBRICANT BASE OIL, METHOD FOR PRODUCTION THEREOF, AND LUBRICANT OIL COMPOSITION Technical Field [0001] The present invention relates to a lubricating base oil, a process for its production and a lubricating oil composition. Background Art [0002] In the field of lubricating oils, additives such as pour point depressants have conventionally been added to lubricating base oils including highly refined mineral oils, to improve the properties such as the low-temperature viscosity characteristic of the lubricating oils (see Patent documents 1-3, for example). Known processes for production of high-viscosity-index base oils include processes in which stock oils containing natural or synthetic normal paraffins are subjected to lubricating base oil refining by hydrocracking/hydroisomerization (see Patent documents 4-6, for example). [0003] Evaluation standards of the low-temperature viscosity characteristic of lubricating base oils and lubricating oils are generally the pour point, clouding point and freezing point. Methods are also known for evaluating the low-temperature viscosity characteristic based on the lubricating base oils, according to their normal paraffin or isoparaffin contents. [Patent document 1] Japanese Unexamined Patent Publication HEI No. 4-36391 [Patent document 2] Japanese Unexamined Patent Publication HEI No. 4-68082 [Patent document 3] Japanese Unexamined Patent Publication HEI No. 4-120193 [Patent document 4] Japanese Unexamined Patent Publication No. 2005-154760 [Patent document 5] Japanese Patent Public Inspection No. 2006-502298 [Patent document 6] Japanese Patent Public Inspection No. 2002-503754 Disclosure of the Invention Problems to be Solved by the Invention [0004] However, with demands increasing in recent years for improved low-temperature viscosity characteristics of lubricating oils and also both low-temperature viscosity characteristics and viscosity-temperature characteristics, it has been difficult to completely satisfy such demands even when using lubricating base oils judged to have satisfactory low-temperature performance based on conventional evaluation standards. [0005] Including additives in lubricating base oils can result in some improvement in the properties, but this method has had its own restrictions. Pour point depressants, in particular, do not exhibit effects proportional to the amounts in which they are added, and even reduce shear stability when added in increased amounts. [0006] It has also been attempted to optimize the conditions for hydrocracking/hydroisomerization in refining processes for lubricating base oils that make use of hydrocracking/hydroisomerization as mentioned above, from the viewpoint of increasing the isomerization rate from normal paraffins to isoparaffins and improving the low- temperature viscosity characteristic by lowering the viscosity of the lubricating base oil, but because the viscosity-temperature characteristic (especially high-temperature viscosity characteristic) and the low-temperature viscosity characteristic are in an inverse relationship, it has been extremely difficult to achieve both of these. For example, increasing the isomerization rate from normal paraffins to isoparaffins improves the low-temperature viscosity characteristic but results in an unsatisfactory viscosity-temperature characteristic, including a reduced viscosity index. The fact that the above-mentioned standards such as pour point and freezing point are often unsuitable for evaluating the low-temperature viscosity characteristic of lubricating base oils is another factor that impedes optimization of the hydrocracking/hydroisomerization conditions. [0007] The present invention has been accomplished in light of these circumstances, and it is an object of the invention to provide a lubricating base oil capable of exhibiting high levels of both viscosity-temperature characteristic and low-temperature viscosity characteristic, as well as a process for its production, and a lubricating oil composition comprising the lubricating base oil. Means for Solving the Problems [0008] In order to solve the problems described above, the invention provides a lubricating base oil characterized by having an urea adduct value of not greater than 4 % by mass and a viscosity index of 100 or greater. [0009] The urea adduct value according to the invention is measured by the following method. A 100 g weighed portion of sample oil (lubricating base oil) is placed in a round bottom flask, 200 g of urea, 360 ml of toluene and 40 ml of methanol are added and the mixture is stirred at room temperature for 6 hours. This produces white particulate crystals as urea adduct in the reaction mixture. The reaction mixture is filtered with a 1 micron filter to obtain the produced white particulate crystals, and the crystals are washed 6 times with 50 ml of toluene. The recovered white crystals are placed in a flask, 300 ml of purified water and 300 ml of toluene are added and the mixture is stirred at 80°C for 1 hour. The aqueous phase is separated and removed with a separatory funnel, and the toluene phase is washed 3 times with 300 ml of purified water. After dewatering treatment of the toluene phase by addition of a desiccant (sodium sulfate), the toluene is distilled off. The proportion (mass percentage) of urea adduct obtained in this manner with respect to the sample oil is defined as the urea adduct value. [0010] The viscosity index according to the invention, and the 40°C or 100°C dynamic viscosity mentioned hereunder, are the viscosity index and 40°C or 100°C dynamic viscosity as measured according to JIS K 2283-1993. [0011] According to the lubricating base oil of the invention, the urea adduct value and viscosity index satisfy the respective conditions specified above, thereby allowing high levels of both viscosity-temperature characteristic and low-temperature viscosity characteristic to be obtained. When an additive such as a pour point depressant is added to the lubricating base oil of the invention, the effect of its addition is exhibited more effectively. Thus, the lubricating base oil of the invention is highly useful as a lubricating base oil that can meet recent demands in terms of both low-temperature viscosity characteristic and viscosity-temperature characteristic. In addition, according to the lubricating base oil of the invention it is possible to reduce viscosity resistance and stirring resistance in a practical temperature range due to its aforementioned superior viscosity-temperature characteristic. In particular, the lubricating base oil of the invention can exhibit this effect by significantly reducing viscosity resistance and stirring resistance under low temperature conditions of 0°C and below, and it is therefore highly useful for reducing energy loss and achieving energy savings in devices in which the lubricating base oil is applied. [0012] While efforts are being made to improve the isomerization rate from normal paraffins to isoparaffins in conventional refining processes for lubricating base oils by hydrocracking and hydroisomerization, as mentioned above, the present inventors have found that it is difficult to satisfactorily improve the low-temperature viscosity characteristic simply by reducing the residual amount of normal paraffins. That is, although the isoparaffins produced by hydrocracking and hydroisomerization also contain components that adversely affect the low-temperature viscosity characteristic, this fact has not been fully appreciated in the conventional methods of evaluation. Methods such as gas chromatography (GC) and NMR are also applied for analysis of normal paraffins and isoparaffins, but using these analysis methods for separation and identification of the components in isoparaffins that adversely affect the low-temperature viscosity characteristic involves complicated procedures and is time-consuming, making them ineffective for practical use. [0013] With measurement of the urea adduct value according to the invention, on the other hand, it is possible to accomplish precise and reliable collection of components in isoparaffins that can adversely affect the low-temperature viscosity characteristic, as well as normal paraffins when normal paraffins are residually present in the lubricating base oil, and it is therefore an excellent evaluation standard of the low-temperature viscosity characteristic of lubricating base oils. The present inventors have confirmed that when analysis is conducted using GC and NMR, the main urea adducts are urea adducts of normal paraffins and of isoparaffins with 6 or more carbon atoms from the end of the main chain to the point of branching. [0014] As an example of a preferred embodiment of the lubricating base oil of the invention, there may be mentioned a lubricating base oil with an urea adduct value of not greater than 4 % by mass, a viscosity index of 130 or greater and a NOACK evaporation amount of not greater than 15 % by mass. [0015] As another preferred embodiment of the lubricating base oil of the invention, there may be mentioned a lubricating base oil with an urea adduct value of not greater than 4 % by mass, a viscosity index of 130 or greater, a -35°C CCS viscosity of not greater than 2000 mPa-s and a product of the 40°C dynamic viscosity (units: mm2/s) and NOACK evaporation amount (units: % by mass) of not greater than 250. [0016] Moreover, the invention provides a process for production of a lubricating base oil characterized by comprising a step of hydrocracking/hydroisomerization of a stock oil containing normal paraffins, until the obtained treatment product has an urea adduct value of not greater than 4 % by mass and a viscosity index of 100 or greater. [0017] According to the process for production of a lubricating base oil according to the invention, it is possible to reliably obtain a lubricating base oil with high levels of both viscosity-temperature characteristic and low-temperature viscosity characteristic, by hydrocracking/hydroisomerization of a stock oil containing normal paraffins until the obtained treatment product has an urea adduct value of not greater than 4 % by mass and a viscosity index of 100 or greater. [0018] As an example of a preferred embodiment of the process for production of a lubricating base oil according to the invention, there may be mentioned a process for production of a lubricating base oil comprising a step of hydrocracking/hydroisomerization of a stock oil containing normal paraffins, until the urea adduct value of the obtained treatment product is not greater than 4 % by mass, the viscosity index is 130 or greater and the NOACK evaporation amount is not greater than 15 % by mass. [0019] As another preferred embodiment of the process for production of a lubricating base oil according to the invention there may be mentioned a process for production of a lubricating base oil comprising a step of hydrocracking/hydroisomerization of a stock oil containing normal paraffins, until the urea adduct value of the obtained treatment product is not greater than 4 % by mass, the viscosity index is 130 or greater, the -35°C CCS viscosity is not greater than 2000 mPa-s, and the product of the 40°C dynamic viscosity (units: mm2/s) and the NOACK evaporation amount (units: % by mass) is not greater than 250. [0020] In the process for production of a lubricating base oil according to the invention, it is preferred for the stock oil to containing at least 50 % by mass slack wax obtained by solvent dewaxing of the lubricating base oil. [0021] The invention still further provides a lubricating oil composition characterized by comprising the aforementioned lubricating base oil of the invention. [0022] Since a lubricating oil composition according to the invention contains a lubricating base oil of the invention having the excellent properties described above, it is useful as a lubricating oil composition capable of exhibiting high levels of both viscosity-temperature characteristic and low-temperature viscosity characteristic. Since the effects of adding additives to the lubricating base oil of the invention can be effectively exhibited, as explained above, various additives may be optimally added to the lubricating oil composition of the invention. Effect of the Invention [0023] According to the invention there are provided a lubricating base oil capable of exhibiting high levels of both viscosity-temperature characteristic and low-temperature viscosity characteristic, as well as a process for its production, and a lubricating oil composition comprising the lubricating base oil. Best Mode for Carrying Out the Invention [0024] Preferred embodiments of the invention will now be described in detail. [0025] The lubricating base oil of the invention has an urea adduct value of not greater than 4 % by mass and a viscosity index of 100 or greater. [0026] From the viewpoint of improving the low-temperature viscosity characteristic without impairing the viscosity-temperature characteristic, the urea adduct value of the lubricating base oil of the invention must be not greater than 4 % by mass as mentioned above, but it is preferably not greater than 3.5 % by mass, more preferably not greater than 3 % by mass and even more preferably not greater than 2.5 % by mass. The urea adduct value of the lubricating base oil may even be 0 % by mass. However, it is preferably 0.1 % by mass or greater, more preferably 0.5 % by mass or greater and particularly preferably 0.8 % by mass or greater, from the viewpoint of obtaining a lubricating base oil with a sufficient low-temperature viscosity characteristic and higher viscosity index, and also of relaxing the dewaxing conditions for increased economy. [0027] From the viewpoint of improving the viscosity-temperature characteristic, the viscosity index of the lubricating base oil of the invention must be 100 or greater as mentioned above, but it is preferably 110 or greater, more preferably 120 or greater, even more preferably 130 or greater and particularly preferably 140 or greater. [0028] The stock oil used for production of the lubricating base oil of the invention may include normal paraffins or normal paraffin-containing wax. The stock oil may be a mineral oil or a synthetic oil, or a mixture of two or more thereof. [0029] The stock oil used for the invention preferably is a wax-containing starting material that boils in the range of lubricating oils according to ASTM D86 or ASTM D2887. The wax content of the stock oil is preferably between 50 % by mass and 100 % by mass based on the total mass of the stock oil. The wax content of the starting material can be measured by a method of analysis such as nuclear magnetic resonance spectroscopy (ASTM D5292), correlative ring analysis (n-d-M) (ASTM D3238) or the solvent method (ASTM D3235). [0030] As examples of wax-containing starting materials there may be mentioned oils derived from solvent refining methods, such as raffinates, partial solvent dewaxed oils, deasphalted oils, distillates, vacuum gas oils, coker gas oils, slack waxes, foot oil, Fischer-Tropsch waxes and the like, among which slack waxes and Fischer-Tropsch waxes are preferred. [0031] Slack wax is typically derived from hydrocarbon starting materials by solvent or propane dewaxing. Slack waxes may contain residual oil, but the residual oil can be removed by deoiling. Foot oil corresponds to deoiled slack wax. [0032] Fischer-Tropsch waxes are produced by so-called Fischer-Tropsch synthesis. [0033] Commercial normal paraffin-containing stock oils are also available. Specifically, there may be mentioned Paraflint 80 (hydrogenated Fischer-Tropsch wax) and Shell MDS Waxy Raffmate (hydrogenated and partially isomerized heart-cut distilled synthetic wax raffmate). [0034] Stock oil from solvent extraction is obtained by feeding a high boiling point petroleum fraction from atmospheric distillation to a vacuum distillation apparatus and subjecting the distillation fraction to solvent extraction. The residue from vacuum distillation may also be deasphalted. In solvent extraction methods, the aromatic components are dissolved in the extracted phase while leaving the more paraffmic components in the raffmate phase. Naphthenes are distributed in the extracted phase and raffinate phase. The preferred solvents for solvent extraction are phenols, furfurals and N-methylpyrrolidone. By controlling the solvent/oil ratio, extraction temperature and method of contacting the solvent with the distillate to be extracted, it is possible to control the degree of separation between the extract phase and raffinate phase. There may also be used as the starting material a bottom fraction obtained from a hydrotreatment apparatus, using a hydrotreatment apparatus with higher hydrocracking performance. [0035] The lubricating base oil of the invention may be obtained through a step of hydrocracking/hydroisomerization of the stock oil until the treatment product has an urea adduct value of not greater than 4 % by mass and a viscosity index of 100 or greater. The hydrocracking/hydroisomerization step is not particularly restricted so long as it satisfies the aforementioned conditions for the urea adduct value and viscosity index of the treatment product. A preferred hydrocracking/hydroisomerization step according to the invention comprises a first step in which a normal paraffin-containing stock oil is subjected to hydrotreatment using a hydrotreatment catalyst, a second step in which the treatment product obtained from the first step is subjected to hydrodewaxing using a hydrodewaxing catalyst, and a third step in which the treatment product obtained from the second step is subjected to hydrorefining using a hydrorefining catalyst. [0036] Conventional hydrocracking/hydroisomerization also includes a hydrotreatment step in an early stage of the hydrodewaxing step, for the purpose of desulfurization and denitrification to prevent poisoning of the hydrodewaxing catalyst. In contrast, the first step (hydrotreatment step) according to the invention is carried out to decompose a portion (for example, about 10 % by mass and preferably 1-10 % by mass) of the normal paraffins in the stock oil at an early stage of the second step (hydrodewaxing step), thus allowing desulfurization and denitrification in the first step as well, although the purpose differs from that of conventional hydrotreatment. The first step is preferred in order to reliably limit the urea adduct value of the treatment product obtained after the third step (the lubricating base oil) to not greater than 4 % by mass. [0037] As hydrogenation catalysts to be used in the first step there may be mentioned catalysts containing Group 6 metals and Group 8-10 metals, as well as mixtures thereof. As preferred metals there may be mentioned nickel, tungsten, molybdenum and cobalt, and mixtures thereof. The hydrogenation catalyst may be used in a form with the aforementioned metals supported on a heat resistant metal oxide carrier, and normally the metal will be present on the carrier as an oxide or sulfide. When a mixture of metals is used, it may be used as a bulk metal catalyst with an amount of metal of at least 30 % by mass based on the total mass of the catalyst. The metal oxide carrier may be an oxide such as silica, alumina, silica-alumina or titania, with alumina being preferred. Preferred alumina is y or p porous alumina. The loading mass of the metal is preferably 0.5-35 % by mass based on the total mass of the catalyst. When a mixture of a metal of Group 9-10 and a metal of Group 6 is used, preferably the metal of Group 9 or 10 is present in an amount of 0.1-5 % by mass and the metal of Group 6 is present in an amount of 5-30 % by mass based on the total mass of the catalyst. The loading mass of the metal may be measured by atomic absorption spectrophotometry or inductively coupled plasma emission spectroscopy, or the individual metals may be measured by other ASTM methods. [0038] The acidity of the metal oxide carrier can be controlled by controlling the addition of additives and the nature of the metal oxide carrier (for example, controlling the amount of silica incorporated in a silica-alumina carrier). As examples of additives there may be mentioned halogens, especially fluorine, and phosphorus, boron, yttria, alkali metals, alkaline earth metals, rare earth oxides and magnesia. Co-catalysts such as halogens generally raise the acidity of metal oxide carriers, but weakly basic additives such as yttria and magnesia can be used to lower the acidity of the carrier. [0039] As regards the hydrotreatment conditions, the treatment temperature is preferably 150-450°C and more preferably 200-400°C, the hydrogen partial pressure is preferably 1400-20000 kPa and more preferably 2800-14000 kPa, the liquid hourly space velocity (LHSV) is preferably 0.1-10 hr"1 and more preferably 0.1-5 hr"1, and the hydrogen/oil ratio is preferably 50-1780 m3/m3 and more preferably 89-890 m /m . These conditions are only for example, and the hydrotreatment conditions in the first step may be appropriately selected depending on difference of starting materials, catalysts and apparatuses, in order to obtain the specified urea adduct value and viscosity index for the treatment product obtained after the third step. [0040] The treatment product obtained by hydrotreatment in the first step may be directly supplied to the second step, but a step of stripping or distillation of the treatment product and separating removal of the gas product from the treatment product (liquid product) is preferably conducted between the first step and second step. This can reduce the nitrogen and sulfur contents in the treatment product to levels that will not affect prolonged use of the hydrodewaxing catalyst in the second step. The main objects of separating removal by stripping and the like are gaseous contaminants such as hydrogen sulfide and ammonia, and stripping can be accomplished by ordinary means such as a flash drum, distiller or the like. [0041] When the hydrotreatment conditions in the first step are mild, residual polycyclic aromatic components can potentially remain depending on the starting material used, and such contaminants may be removed by hydrorefining in the third step. [0042] The hydrodewaxing catalyst used in the second step may contain crystalline or amorphous materials. As examples of crystalline materials there may be mentioned molecular sieves having 10- or 12-membered ring channels, composed mainly of aluminosilicates (zeolite) or silicoaluminophosphates (SAPO). As specific examples of zeolites there may be mentioned ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57, ferrierite, ITQ-13, MCM-68, MCM-71 and the like. ECR-42 may be mentioned as an example of an aluminophosphate. As examples of molecular sieves there may be mentioned zeolite beta and MCM-68. Among the above there are preferably used one or more selected from among ZSM-48, ZSM-22 and ZSM-23, with ZSM-48 being particularly preferred. The molecular sieves are preferably hydrogen-type. Reduction of the hydrodewaxing catalyst may occur at the time of hydrodewaxing, but alternatively a hydrodewaxing catalyst that has been previously subjected to reduction treatment may be used for the hydrodewaxing. [0043] As amorphous materials for the hydrodewaxing catalyst there may be mentioned alumina doped with Group 3 metals, fluorinated alumina, silica-alumina, fluorinated silica-alumina, silica-alumina and the like. [0044] A preferred embodiment of the dewaxing catalyst is a bifunctional catalyst, i.e. one carrying a metal hydrogenated component which is at least one metal of Group 6, at least one metal of Groups 8-10, or a mixture thereof. Preferred metals are precious metals of Groups 9-10, such as Pt, Pd or mixtures thereof. Such metals are supported at preferably 0.1-30 % by mass based on the total mass of the catalyst. The method for preparation of the catalyst and loading of the metal may be, for example, an ion exchange method or impregnation method using a decomposable metal salt. [0045] When molecular sieves are used, they may be compounded with a binder material that is heat resistant under the hydrodewaxing conditions, or they may be binderless (self-binding). As binder materials there may be mentioned inorganic oxides, including silica, alumina, silica-alumina, two-component combinations of silica with other metal oxides such as titania, magnesia, thoria and zirconia, and three-containing combinations of oxides such as silica-alumina-thoria, silica-alumina-magnesia and the like. The amount of molecular sieves in the hydrodewaxing catalyst is preferably 10-100 % by mass and more preferably 35-100 % by mass based on the total mass of the catalyst. The hydrodewaxing catalyst may be formed by a method such as spray-drying or extrusion. The hydrodewaxing catalyst may be used in sulfided or non-sulfided form, although a sulfided form is preferred. [0046] As regards the hydrodewaxing conditions, the temperature is preferably 250-400°C and more preferably 275-350°C, the hydrogen partial pressure is preferably 791-20786 kPa (100-3000 psig) and more preferably 1480-17339 kPa (200-2500 psig), the liquid hourly space velocity is preferably 0.1-10 hr"1 and more preferably 0.1-5 hr"1, and the hydrogen/oil ratio is preferably 45-1780 m3/m3 (250-10000 scf/B) and more preferably 89-890 m3/m3 (500-5000 scf/B). These conditions are only for example, and the hydrodewaxing conditions in the second step may be appropriately selected depending on difference of starting materials, catalysts and apparatuses, in order to obtain the specified urea adduct value and viscosity index for the treatment product obtained after the third step. [0047] The treatment product that has been hydrodewaxed in the second step is then supplied to hydrorefining in the third step. Hydrorefining is a form of mild hydrotreatment aimed at removing residual heteroatoms and color components while also saturating the olefins and residual aromatic compounds by hydrogenation. The hydrorefining in the third step may be carried out in a cascade fashion with the dewaxing step. [0048] The hydrorefining catalyst used in the third step is preferably one comprising a Group 6 metal, a Group 8-10 metal or a mixture thereof supported on a metal oxide carrier. As preferred metals there may be mentioned precious metals, and especially platinum, palladium and mixtures thereof. When a mixture of metals is used, it may be used as a bulk metal catalyst with an amount of metal of 30 % by mass or greater based on the mass of the catalyst. The metal content of the catalyst is preferably not greater than 20 % by mass of non-precious metals and preferably not greater than 1 % by mass of precious metals. The metal oxide carrier may be either an amorphous or crystalline oxide. Specifically, there may be mentioned low acidic oxides such as silica, alumina, silica-alumina and titania, with alumina being preferred. From the viewpoint of saturation of aromatic compounds, it is preferred to use a hydrorefining catalyst comprising a metal with a relatively powerful hydrogenating function supported on a porous carrier. [0049] As preferred hydrorefining catalysts there may be mentioned meso-microporous materials belonging to the M41S class or M41S line catalysts. M41S line catalysts are meso-microporous materials with high silica contents, and specifically there may be mentioned MCM-41, MCM-48 and MCM-50. The hydrorefining catalyst has a pore size of 15-100 A, and MCM-41 is particularly preferred. MCM-41 is an inorganic porous non-laminar phase with a hexagonal configuration and pores of uniform size. The physical structure of MCM-41 is straw-like bundles with straw openings (pore cell diameters) in the range of 15- 100 angstroms. MCM-48 has cubic symmetry, while MCM-50 has a laminar structure. MCM-41 may also have a structure with pore openings having different meso-microporous ranges. The meso-microporous material may contain metal hydrogenated components consisting of one or more Group 8, 9 or 10 metals, and preferred as metal hydrogenated components are precious metals, especially Group 10 precious metals, and most preferably Pt, Pd or their mixtures. [0050] As regards the hydrorefining conditions, the temperature is preferably 150-350°C and more preferably 180-250°C, the total pressure is preferably 2859-20786 kPa (approximately 400-3000 psig), the liquid hourly space velocity is preferably 0.1-5 hr"1 and more preferably 0.5-3 hr"1, and the hydrogen/oil ratio is preferably 44.5-1780 m3/m3 (250-10000 scfTB). These conditions are only for example, and the hydrorefining conditions in the third step may be appropriately selected depending on difference of starting materials and treatment apparatuses, so that the urea adduct value and viscosity index for the treatment product obtained after the third step satisfy the respective conditions specified above. [0051] The treatment product obtained after the third step may be subjected to distillation or the like as necessary for separating removal of certain components. [0052] The lubricating base oil of the invention obtained by the production process described above is not restricted in terms of its other properties so long as the urea adduct value and viscosity index satisfy their respective conditions, but the lubricating base oil of the invention preferably also satisfies the conditions specified below. [0053] The saturated component content of the lubricating base oil of the invention is preferably 90 % by mass or greater, more preferably 93 % by mass or greater and even more preferably 95 % by mass or greater based on the total mass of the lubricating base oil. The proportion of cyclic saturated components among the saturated components is preferably 0.1-50 % by mass, more preferably 0.5-40 % by mass, even more preferably 1-30 % by mass and particularly preferably 5-20 % by mass. If the saturated component content and proportion of cyclic saturated components among the saturated components both satisfy these respective conditions, it will be possible to achieve adequate levels for the viscosity-temperature characteristic and thermal and oxidation stability, while additives added to the lubricating base oil will be kept in a sufficiently stable dissolved state in the lubricating base oil so that the functions of the additives can be exhibited at a higher level. In addition, a saturated component content and proportion of cyclic saturated components among the saturated components satisfying the aforementioned conditions can improve the frictional properties of the lubricating base oil itself, resulting in a greater friction reducing effect and thus increased energy savings. [0054] If the saturated component content is less than 90 % by mass, the viscosity-temperature characteristic, thermal and oxidation stability and frictional properties will tend to be inadequate. If the proportion of cyclic saturated components among the saturated components is less than 0.1 % by mass, the solubility of the additives included in the lubricating base oil will be insufficient and the effective amount of additives kept dissolved in the lubricating base oil will be reduced, making it impossible to effectively achieve the function of the additives. If the proportion of cyclic saturated components among the saturated components is greater than 50 % by mass, the efficacy of additives included in the lubricating base oil will tend to be reduced. [0055] According to the invention, a proportion of 0.1-50 % by mass cyclic saturated components among the saturated components is equivalent to 99.9-50 % by mass acyclic saturated components among the saturated components. Both normal paraffins and isoparaffms are included by the term "acyclic saturated components". The proportions of normal paraffins and isoparaffms in the lubricating base oil of the invention are not particularly restricted so long as the urea adduct value satisfies the condition specified above, but the proportion of isoparaffms is preferably 50-99.9 % by mass, more preferably 60-99.9 % by mass, even more preferably 70-99.9 % by mass and particularly preferably 80-99.9 % by mass based on the total mass of the lubricating base oil. If the proportion of isoparaffms in the lubricating base oil satisfies the aforementioned conditions it will be possible to further improve the viscosity-temperature characteristic and thermal and oxidation stability, while additives added to the lubricating base oil will be kept in a sufficiently stable dissolved state in the lubricating base oil so that the functions of the additives can be exhibited at an even higher level. [0056] The saturated component content for the purpose of the invention is the value measured according to ASTM D 2007-93 (units: % by mass). [0057] The proportions of the cyclic saturated components and acyclic saturated components among the saturated components for the purpose of the invention are the naphthene portion (measurement of monocyclic- hexacyclic naphthenes, units: % by mass) and alkane portion (units: % by mass), respectively, both measured according to ASTM D 2786-91. [0058] The proportion of normal paraffins in the lubricating base oil for the purpose of the invention is the value obtained by analyzing saturated components separated and fractionated by the method of ASTM D 2007-93 by gas chromatography under the following conditions, and calculating the value obtained by identifying and quantifying the proportion of normal paraffins among those saturated components, with respect to the total mass of the lubricating base oil. For identification and quantitation, a C5-50 normal paraffin mixture sample is used as the reference sample, and the normal paraffin content among the saturated components is determined as the proportion of the total of the peak areas corresponding to each normal paraffin, with respect to the total peak area of the chromatogram (subtracting the peak area for the diluent). (Gas chromatography conditions) Column: Liquid phase nonpolar column (length: 25 mm, inner diameter: 0.3 mrncp, liquid phase film thickness: 0.1 urn), temperature elevating conditions: 50°C-400°C (temperature-elevating rate: 10°C/min). Carrier gas: helium (linear speed: 40 cm/min) Split ratio: 90/1 Sample injection rate: 0.5 uL (injection rate of sample diluted 20-fold with carbon disulfide). [0059] The proportion of isoparaffins in the lubricating base oil is the value of the difference between the acyclic saturated components among the saturated components and the normal paraffins among the saturated components, based on the total mass of the lubricating base oil. [0060] Other methods may be used for separation of the saturated components or for compositional analysis of the cyclic saturated components and acyclic saturated components, so long as they provide similar results. As examples of other methods there may be mentioned the method according to ASTM D 2425-93, the method according to ASTM D 2549-91, methods of high performance liquid chromatography (HPLC), and modified forms of these methods. [0061] When the bottom fraction obtained from a hydrotreatment apparatus is used as the starting material for the lubricating base oil of the invention, the obtained base oil will have a saturated component content of 90 % by mass or greater, a proportion of cyclic saturated components in the saturated components of 30-50 % by mass, a proportion of acyclic saturated components in the saturated components of 50-70 % by mass, a proportion of isoparaffins in the lubricating base oil of 40-70 % by mass and a viscosity index of 100-135 and preferably 120-130, but if the urea adduct value satisfies the conditions specified above it will be possible to obtain a lubricating oil composition with the effect of the invention, i.e. an excellent low-temperature viscosity characteristic wherein the -40°C MRV viscosity is not greater than 20000 mPa-s and especially not greater than 10000 mPa-s. When a slack wax or Fischer-Tropsch wax having a high wax content (for example, a normal paraffin content of 50 % by mass or greater) is used as the starting material for the lubricating base oil of the invention, the obtained base oil will have a saturated component content of 90 % by mass or greater, a proportion of cyclic saturated components in the saturated components of 0.1-40 % by mass, a proportion of acyclic saturated components in the saturated components of 60-99.9 % by mass, a proportion of isoparaffins in the lubricating base oil of 60-99.9 % by mass and a viscosity index of 100-170 and preferably 135-160, but if the urea adduct value satisfies the conditions specified above it will be possible to obtain a lubricating oil composition with very excellent properties in terms of the effect of the invention, and especially the high viscosity index and low-temperature viscosity characteristic, wherein the -40°C MRV viscosity is not greater than 12000 mPa-s and especially not greater than 7000 mPa-s. [0062] If the 20°C refractive index is represented as n2o and the 100°C dynamic viscosity is represented as kvlOO, the value of n2o - 0.002 * kvlOO for the lubricating base oil of the invention is preferably 1.435-1.450, more preferably 1.440-1.449, even more preferably 1.442-1.448 and yet more preferably 1.444-1.447. If n20 - 0.002 x kvlOO is within the range specified above it will be possible to achieve an excellent viscosity-temperature characteristic and thermal and oxidation stability, while additives added to the lubricating base oil will be kept in a sufficiently stable dissolved state in the lubricating base oil so that the functions of the additives can be exhibited at an even higher level. The n2o - 0.002 x kvlOO value within the aforementioned range can also improve the frictional properties of the lubricating base oil itself, resulting in a greater friction reducing effect and thus increased energy savings. [0063] If the n20 - 0.002 x kvlOO value exceeds the aforementioned upper limit, the viscosity-temperature characteristic, thermal and oxidation stability and frictional properties will tend to be insufficient, and the efficacy of additives when added to the lubricating base oil will tend to be reduced. If the n2o - 0.002 x kvlOO value is less than the aforementioned lower limit, the solubility of the additives included in the lubricating base oil will be insufficient and the effective amount of additives kept dissolved in the lubricating base oil will be reduced, making it impossible to effectively achieve the functions of the additives. [0064] The 20°C refractive index (n2o) for the purpose of the invention is the refractive index measured at 20°C according to ASTM D1218-92. The 100°C dynamic viscosity (kvlOO) for the purpose of the invention is the dynamic viscosity measured at 100°C according to JIS K 2283-1993. [0065] The aromatic content of the lubricating base oil of the invention is preferably not greater than 5 % by mass, more preferably 0.05-3 % by mass, even more preferably 0.1-1 % by mass and particularly preferably 0.1-0.5 % by mass based on the total mass of the lubricating base oil. If the aromatic content exceeds the aforementioned upper limit, the viscosity-temperature characteristic, thermal and oxidation stability, frictional properties, resistance to volatilization and low-temperature viscosity characteristic will tend to be reduced, while the efficacy of additives when added to the lubricating base oil will also tend to be reduced. The lubricating base oil of the invention may be free of aromatic components, but the solubility of additives can be further increased with an aromatic content of 0.05 % by mass or greater. [0066] The aromatic content in this case is the value measured according to ASTM D 2007-93. The aromatic portion normally includes alkylbenzenes and alkylnaphthalenes, as well as anthracene, phenanthrene and their alkylated forms, compounds with four or more condensed benzene rings, and heteroatom-containing aromatic compounds such as pyridines, quinolines, phenols, naphthols and the like. [0067] The %CP value of the lubricating base oil of the invention is preferably 80 or greater, more preferably 82-99, even more preferably 85-98 and particularly preferably 90-97. If the %CP value of the lubricating base oil is less than 80, the viscosity-temperature characteristic, thermal and oxidation stability and frictional properties will tend to be reduced, while the efficacy of additives when added to the lubricating base oil will also tend to be reduced. If the %CP value of the lubricating base oil is greater than 99, on the other hand, the additive solubility will tend to be lower. [0068] The %CN value of the lubricating base oil of the invention is preferably not greater than 20, more preferably not greater than 15, even more preferably 1-12 and particularly preferably 3-10. If the %CN value of the lubricating base oil exceeds 20, the viscosity-temperature characteristic, thermal and oxidation stability and frictional properties will tend to be reduced. If the %CN is less than 1, the additive solubility will tend to be lower. [0069] The %CA value of the lubricating base oil of the invention is preferably not greater than 0.7, more preferably not greater than 0.6 and even more preferably 0.1-0.5. If the %CA value of the lubricating base oil exceeds 0.7, the viscosity-temperature characteristic, thermal and oxidation stability and frictional properties will tend to be reduced. The %CA value of the lubricating base oil of the invention may be zero, but the solubility of additives can be further increased with a %CA value of 0.1 or greater. [0070] The ratio of the %CP and %CN values for the lubricating base oil of the invention is %CP/%CN of preferably 7 or greater, more preferably 7.5 or greater and even more preferably 8 or greater. If the %CP/%CN ratio is less than 7, the viscosity-temperature characteristic, thermal and oxidation stability and frictional properties will tend to be reduced, while the efficacy of additives when added to the lubricating base oil will also tend to be reduced. The %CP/%CN ratio is preferably not greater than 200, more preferably not greater than 100, even more preferably not greater than 50 and particularly preferably not greater than 25. The additive solubility can be further increased if the %CP/%CN ratio is not greater than 200. [0071] The %CP, %CN and %CA values for the purpose of the invention are, respectively, the percentage of paraffinic carbons with respect to total carbon atoms, the percentage of naphthenic carbons with respect to total carbons and the percentage of aromatic carbons with respect to total carbons, as determined by the methods of ASTM D 3238-85 (n-d-M ring analysis). That is, the preferred ranges for %CP, %CN and %CA are based on values determined by these methods, and for example, %CN may be a value exceeding 0 according to these methods even if the lubricating base oil contains no naphthene portion. [0072] The iodine value of the lubricating base oil of the invention is preferably not greater than 0.5, more preferably not greater than 0.3 and even more preferably not greater than 0.15, and although it may be less than 0.01, it is preferably 0.001 or greater and more preferably 0.05 or greater in consideration of economy and achieving a significant effect. Limiting the iodine value of the lubricating base oil to not greater than 0.5 can drastically improve the thermal and oxidation stability. The "iodine value" for the purpose of the invention is the iodine value measured by the indicator titration method according to JIS K 0070, "Acid Values, Saponification Values, Iodine Values, Hydroxyl Values And Unsaponiflcation Values Of Chemical Products". [0073] The sulfur content in the lubricating base oil of the invention will depend on the sulfur content of the starting material. For example, when using a substantially sulfur-free starting material as for synthetic wax components obtained by Fischer-Tropsch reaction, it is possible to obtain a substantially sulfur-free lubricating base oil. When using a sulfur-containing starting material, such as slack wax obtained by a lubricating base oil refining process or microwax obtained by a wax refining process, the sulfur content of the obtained lubricating base oil will normally be 100 ppm by mass or greater. From the viewpoint of further improving the thermal and oxidation stability and reducing sulfur in the lubricating base oil of the invention, the sulfur content is preferably not greater than 10 ppm by mass, more preferably not greater than 5 ppm by mass, and even more preferably not greater than 3 ppm by mass. [0074] From the viewpoint of cost reduction it is preferred to use slack wax or the like as the starting material, in which case the sulfur content of the obtained lubricating base oil is preferably not greater than 50 ppm by mass and more preferably not greater than 10 ppm by mass. The sulfur content for the purpose of the invention is the sulfur content measured according to JIS K 2541-1996. [0075] The nitrogen content in the lubricating base oil of the invention is not particularly restricted, but is preferably not greater than 5 ppm by mass, more preferably not greater than 3 ppm by mass and even more preferably not greater than 1 ppm by mass. If the nitrogen content exceeds 5 ppm by mass, the thermal and oxidation stability will tend to be reduced. The nitrogen content for the purpose of the invention is the nitrogen content measured according to JIS K 2609-1990. [0076] The dynamic viscosity of the lubricating base oil according to the invention, as the 100°C dynamic viscosity, is preferably 1.5-20 mm /s and more preferably 2.0-11 mm Is. A 100°C dynamic viscosity of lower than 1.5 mm /s for the lubricating base oil is not preferred from the standpoint of evaporation loss. If it is attempted to obtain a lubricating base oil having a 100°C dynamic viscosity of greater than 20 mm Is, the yield will be reduced and it will be difficult to increase the cracking severity even when using a heavy wax as the starting material. [0077] According to the invention, a lubricating base oil having a 100°C dynamic viscosity in the following range is preferably used after fractionation by distillation or the like. (I) A lubricating base oil with a 100°C dynamic viscosity of at least 1.5 mm2/s and less than 3.5 mm2/s, and more preferably 2.0-3.0 mm2/s. (II) A lubricating base oil with a 100°C dynamic viscosity of at least 3.0 mm Is and less than 4.5 mm Is, and more preferably 3.5-4.1 mm /s. (III) A lubricating base oil with a 100°C dynamic viscosity of 4.5-20 mm2/s, more preferably 4.8-11 mm2/s and particularly preferably 5.5- 8.0 mm2/s. [0078] The 40°C dynamic viscosity of the lubricating base oil of the 9 9 invention is preferably 6.0-80 mm /s and more preferably 8.0-50 mm /s. According to the invention, a lube-oil distillate having a 40°C dynamic viscosity in the following ranges is preferably used after fractionation by distillation or the like. (IV) A lubricating base oil with a 40°C dynamic viscosity of at least 6.0 9 9 9 mm /s and less than 12 mm /s, and more preferably 8.0-12 mm /s. (V) A lubricating base oil with a 40°C dynamic viscosity of at least 12 9 9 9 mm /s and less than 28 mm /s, and more preferably 13-19 mm /s. (VI) A lubricating base oil with a 40°C dynamic viscosity of 28-50 9 9 mm /s, more preferably 29-45 mm /s and particularly preferably 30-40 mm2/s. [0079] The lubricating base oils (I) and (IV), having urea adduct values and viscosity indexes satisfying the respective conditions specified above, can achieve high levels of both viscosity-temperature characteristic and low-temperature viscosity characteristic compared to conventional lubricating base oils of the same viscosity grade, and in particular they have an excellent low-temperature viscosity characteristic whereby the viscosity resistance or stirring resistance can notably reduced. Moreover, by including a pour point depressant it is possible to lower the -40°C BF viscosity to not greater than 2000 mPa-s. The -40°C BF viscosity is the viscosity measured according to JPI-5S-26-99. [0080] The lubricating base oils (II) and (V), having urea adduct values and viscosity indexes satisfying the respective conditions specified above, can achieve high levels of both the viscosity-temperature characteristic and low-temperature viscosity characteristic compared to conventional lubricating base oils of the same viscosity grade, and in particular they have an excellent low-temperature viscosity characteristic, and superior lubricity and resistance to volatilization. For example, with lubricating base oils (II) and (V) it is possible to lower the -35°C CCS viscosity to not greater than 3000 mPa-s. [0081] The lubricating base oils (III) and (VI), having urea adduct values and viscosity indexes satisfying the respective conditions specified above, can achieve high levels of both the viscosity-temperature characteristic and low-temperature viscosity characteristic compared to conventional lubricating base oils of the same viscosity grade, and in particular they have an excellent low-temperature viscosity characteristic, and superior thermal and oxidation stability, lubricity and resistance to volatilization. [0082] The 20°C refractive index of the lubricating base oil of the invention will depend on the viscosity grade of the lubricating base oil, but the 20°C refractive indexes of the lubricating base oils (I) and (IV) mentioned above are preferably not greater than 1.455, more preferably not greater than 1.453 and even more preferably not greater than 1.451. The 20°C refractive index of the lubricating base oils (II) and (V) is preferably not greater than 1.460, more preferably not greater than 1.457 and even more preferably not greater than 1.455. The 20°C refractive index of the lubricating base oils (III) and (VI) is preferably not greater than 1.465, more preferably not greater than 1.463 and even more preferably not greater than 1.460. If the refractive index exceeds the aforementioned upper limit, the viscosity-temperature characteristic, thermal and oxidation stability, resistance to volatilization and low-temperature viscosity characteristic of the lubricating base oil will tend to be reduced, while the efficacy of additives when added to the lubricating base oil will also tend to be reduced. [0083] The pour point of the lubricating base oil of the invention will depend on the viscosity grade of the lubricating base oil, and for example, the pour point for the lubricating base oils (I) and (IV) is preferably not greater than -10°C, more preferably not greater than -12.5°C and even more preferably not greater than -15°C. The pour point for the lubricating base oils (II) and (V) is preferably not greater than -10°C, more preferably not greater than -15°C and even more preferably not greater than -17.5°C. The pour point for the lubricating base oils (III) and (VI) is preferably not greater than -10°C, more preferably not greater than -12.5°C and even more preferably not greater than -15°C. If the pour point exceeds the upper limit specified above, the low-temperature flow properties of lubricating oils employing the lubricating base oils will tend to be reduced. The pour point for the purpose of the invention is the pour point measured according to JIS K 2269-1987. [0084] The -35°C CCS viscosity of the lubricating base oil of the invention will depend on the viscosity grade of the lubricating base oil, but the -35°C CCS viscosities of the lubricating base oils (I) and (IV) are preferably not greater than 1000 mPa-s. The -35°C CCS viscosity for the lubricating base oils (II) and (V) is preferably not greater than 3000 mPa-s, more preferably not greater than 2400 mPa-s, even more preferably not greater than 2000 mPa-s, even more preferably not greater than 1800 mPa-s and particularly preferably not greater than 1600 mPa-s. The -35°C CCS viscosity for the lubricating base oils (III) and (VI), for example, are preferably not greater than 15000 mPa-s and more preferably not greater than 10000 mPa-s. If the -35°C CCS viscosity exceeds the upper limit specified above, the low-temperature flow properties of lubricating oils employing the lubricating base oils will tend to be reduced. The -35°C CCS viscosity for the purpose of the invention is the viscosity measured according to JIS K 2010-1993. [0085] The -40°C BF viscosity of the lubricating base oil of the invention will depend on the viscosity grade of the lubricating base oil, but the -40°C BF viscosities of the lubricating base oils (I) and (IV), for example, are preferably not greater than 10000 mPa-s, more preferably 8000 mPa-s, and even more preferably not greater than 6000 mPa-s. The -40°C BF viscosities of the lubricating base oils (II) and (V) are preferably not greater than 1500000 mPa-s and more preferably not greater than 1000000 mPa-s. If the -40°C BF viscosity exceeds the upper limit specified above, the low-temperature flow properties of lubricating oils employing the lubricating base oils will tend to be reduced. [0086] The 15°C density (pi5) of the lubricating base oil of the invention will also depend on the viscosity grade of the lubricating base oil, but it is preferably not greater than the value of p as represented by the following formula (1), i.e., pi5p. p = 0.0025 xkvlOO+0.816 (1) [In this equation, kvlOO represents the 100°C dynamic viscosity (mm2/s) of the lubricating base oil.] [0087] If pi5>p, the viscosity-temperature characteristic, thermal and oxidation stability, resistance to volatilization and low-temperature viscosity characteristic of the lubricating base oil will tend to be reduced, while the efficacy of additives when added to the lubricating base oil will also tend to be reduced. [0088] For example, the value of pi5 for lubricating base oils (I) and (IV) is preferably not greater than 0.825 and more preferably not greater than 0.820. The value of pi5 for lubricating base oils (II) and (V) is preferably not greater than 0.835 and more preferably not greater than 0.830. Also, the value of pj5 for lubricating base oils (III) and (VI) is preferably not greater than 0.840 and more preferably not greater than 0.835. [0089] The 15°C density for the purpose of the invention is the density measured at 15°C according to JIS K 2249-1995. [0090] The aniline point (AP (°C)) of the lubricating base oil of the invention will also depend on the viscosity grade of the lubricating base oil, but it is preferably greater than or equal to the value of A as represented by the following formula (2), i.e., AP>A. A = 4.3xkvl00+100 (2) [In this equation, kvlOO represents the 100°C dynamic viscosity (mm /s) of the lubricating base oil.] [0091] If AP