TITLE VISCOSITY MODIFIER FOR LUBRICATING OIL AND LUBRICATING OIL COMPOSITION TECHNICAL FIELD The present invention relates to a viscosity modifier for a lubricating oil and a lubricating oil composition. More particularly, the invention relates to a viscosity modifier for a lubricating oil capable of producing a lubricating oil composition having excellent low-temperature properties and a lubricating oil composition containing the viscosity modifier. BACKGROUND ART The viscosity of petroleum products generally varies greatly with temperature, and for lubricating oils for automobiles, the temperature dependence of the viscosity is desired to be small. In recent years, therefore, an ethylene/a-olef in copolymer has been widely used as a viscosity modifier having an effect of improving viscosity index for the purpose of decreasing the temperature dependence of the lubricating oils. When the surrounding temperature lowers, a wax component in a lubricating oil is crystallized and solidified to make the lubricating oil lose flowability, so that a pour point depressant is also contained in the lubricating oil to depress the solidification temperature. The pour point depressant functions to inhibit formation of a three-dimensional network attributed to crystallization of the wax component in the lubricating oil and to depress the pour point of the lubricating oil. Of the low-temperature properties of a lubricating oil containing a viscosity modifier having an effect of improving viscosity index and a pour point depressant, the viscosity at a high shear rate is determined by compatibility of a lubricating oil base with the viscosity modifier, but on the other hand, the viscosity at a low shear rate is greatly influenced by the pour point depressant. It is known that when an ethylene/a-olefin copolymer having specific composition is used as a viscosity modifier, the effect of the pour point depressant is markedly reduced because of an interaction between the copolymer and the pour point depressant (see U.S. Patents No. 3,697,429 and No. 3,551,336). Accordingly, the viscosity modifier to be blended with a lubricating oil which is required to have particularly excellent low-temperature properties is desired to exhibit an excellent effect of improving viscosity index and not to inhibit the function of the pour point depressant. As a viscosity modifier satisfying such requirements, an ethylene/a-olefin copolymer, which has an ununiform distribution of ethylene units and a-olefin units in the molecules, an ethylene content of 30 to 80 % by weight, a weight-average molecular weight of 20, 000 to 750, 000 and Mw/Mn of less than 2, is disclosed in Japanese Patent Publication No. 96624/1994. The present inventors have earnestly studied under such circumstances as mentioned above, and as a result, they have found that an ethylene/propylene copolymer having a density, a molecular weight, a molecular weight distribution and a melting point in the specific ranges and having a specific relation between the density and the melting point and an ethylene/propylene copolymer having an ethylene content, a molecular weight, a molecular weight distribution and a melt ing point in the specific ranges and having a specific relation between the ethylene content and the melting point have an excellent effect of improving viscosity index and do not inhibit the function of the pour point depressant. Based on the finding, the present invention has been accomplished. For reference, the ethylene/a-olefincopolymer disclosed in the above publication satisfies neither the relation between the ethylene content and the melting point of the ethylene/propylene copolymer nor the relation between the density and the melting point of the ethylene/propylene copolymer specified by the present invention. OBJECT OF THE INVENTION It is an object of the present invention to provide a viscosity modifier for a lubricating oil, which comprises a specific ethylene/propylene copolymer and by which a lubricating oil composition having excellent low-temperature properties can be obtained, and to provide a lubricating oil composition containing the viscosity modifier and having excellent low-temperature properties. DISCLOSURE OF THE INVENTION One embodiment of the viscositymodif ier for a lubricating oil according to the present invention comprises an ethylene/propylene copolymer (A) having the following properties (a-1) to (a-5): (a-1) the density is in the range of 857 to 882 kg/m3, (a-2) the weight-average molecular weight as measured by gel permeation chromatography is in the range of 80,000 to 400,000 in terms of polystyrene, (a-3) Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight), which is an indication of a molecular weight distribution, is not more than 2.3, (a-4) the melting point as measured by a differential scanning calorimeter is in the range of 15 to 60°C, and (a-5) the density (D (kg/m3) ) and the melting point (Tm (°C)) as measured by a differential scanning calorimeter satisfy the following relation (I) Tm ^ 1.247x0-1037 (I). Another embodiment of the viscosity modifier for a lubricating oil according to the present invention comprises an ethylene/propylene copolymer (B) having the following properties (b-1) to (b-5): (b-1) the content of recurring units derived f romethylene is in the range of 70 to 79 % by weight, (b-2) the weight-average molecular weight as measured by gel permeation chromatography is not less than 80,000 and less than 250,000 in terms of polystyrene, (b-3) Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight), which is an indication of a molecular weight distribution, is not more than 2.3, (b-4) the melting point as measured by a differential scanning calorimeter is in the range of 15 to 60°C, and (b-5) the content (E (% by weight) ) of recurring units derived f romethylene and themeltingpoint (Tm (°C) ) asmeasured by a differential scanning calorimeter satisfy the following relation (II) 3.44xE-206 ≥ Tm(II). A further embodiment of the viscosity modifier for a lubricating oil according to the present invention comprises an ethylene/propylene copolymer (C) having the following properties (c-1) to (c-5): (c-1) the content of recurring units derived f romethylene is in the range of 70 to 79 % by weight, (c-2) the weight-average molecular weight as measured by gel permeation chromatography is in the range of 250,000 to 400,000 in terms of polystyrene, (c-3) Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight), which is an indication of a molecular weight distribution, is not more than 2.3, (c-4) the melting point as measured by a differential scanning calorimeter is in the range of 15 to 60°C, and (c-5) the content (E (% by weight)) of recurring units derivedfromethyleneandthemeltingpoint (Tm(°C)) asmeasured by a differential scanning calorimeter satisfy the following relation (III) 3.44xE-204 ≥ Tm(III). When the viscosity modifier for a lubricating oil according to the present invention isblended with a lubricating oil, a lubricating oil having excellent low-temperature properties can be obtained. Embodiments of the lubricating oil composition according to the present invention include: a lubricating oil composition comprising (A) the ethylene/propylene copolymer having the above properties (a-1) to (a-5), and (D) a lubricating oil base, wherein the ethylene/propylene copolymer (A) is contained in an amount of 1 to 20 % by weight; a lubricating oil composition comprising (B) the ethylene/propylene copolymer having the above properties (b-1) to (b-5), and (D) a lubricating oil base, wherein the ethylene/propylene copolymer (B) is contained in an amount of 1 to 20 % by weight; and a lubricating oil composition comprising (C) an ethylene/propylene copolymer having the above properties (c-1) to (c-5), and (D) a lubricating oil base, wherein the ethylene/propylene copolymer (C) is contained in an amount of 1 to 20 % by weight. Other embodiments of the lubricating oil composition according to the present invention include: a lubricating oil composition comprising (A) the ethylene/propylene copolymer having the above properties (a-1) to (a-5), (D) a lubricating oil base, and (E) a pour point depressant, wherein the ethylene/propylenecopolymer (A) is contained in an amount of 0.1 to 5 % by weight and the pour point depressant (E) is contained in an amount of 0.05 to 5 % by weight; a lubricating oil composition comprising (B) the ethylene/propylene copolymer having the above properties (b-1) to (b-5), (D) a lubricating oil base, and (E) a pour point depressant, wherein the ethylene/propylene copolymer (B) is contained in an amount of 0.1 to 5 % by weight and the pour point depressant (E) is contained in an amount of 0.05 to 5 % by weight; and a lubricating oil composition comprising (C) an ethylene/propylene copolymer having the above properties (c-1) to (c-5), (D) a lubricating oil base, and (E) a pour point depressant, wherein the ethylene/propylene copolymer (C) is contained in an amount of 0 .1 to 5 % by weight and the pour point depressant (E) is contained in an amount of 0.05 to 5 % by weight. The lubricating oil composition according to the present invention has excellent low-temperature properties. BEST MODE FOR CARRYING OUT THE INVENTION The viscosity modifier for a lubricating oil and the lubricating oil composition according to the invention are described in more detail hereinafter. Viscosity modifier for lubricating oil An embodiment of the viscosity modifier for a lubricating oil according to the invention comprises the following ethylene/propylene copolymer (A). Ethylene/propylene copolymer (A) The ethylene/propylene copolymer (A) comprises recurring units derived from ethylene and recurring units derived from propylene. Although the ethylene content in the ethylene/propylene copolymer (A) is not specifically limited as far as the density is within such range as mentioned later, it is in the range of usually 70 to 79 % by weight, preferably 71 to 78 % by weight, more preferably 72 to 78 % by weight, still more preferably 73 to 77 % by weight, particularly preferably 75 to 77 % by weight. The residue is a content of recurring units derived from propylene and the like. In the present invention, the ethylene content in the ethylene/propylene copolymer is determined by l-^C-NMR ^n accordance with the method described in "Macromolecule Analysis Handbook" (Society of Japan Analytical Chemistry, edited by Macromolecule Analytical Research Meet ing, published by Kinokuniya Shoten). In the ethylene/propylene copolymer (A) , recurring units derived from at least one monomer selected from a-olefins of 4 to 20 carbon atoms, cycloolefins, polyenes and aromatic olefins (hereinafter sometimes referred to as "other monomers") may be contained in amounts of, for example, not more than 5 % by weight, preferably not more than 1 % by weight, within limits not detrimental to the objects of the invention. The ethylene/propylene copolymer (A) has a density of 857 to 882 kg/m3, preferably 859 to 880 kg/m3, more preferably 860 to 880 kg/m3, still more preferably 864 to 875 kg/m3, particularly preferably 868 to 875 kg/m3. When the density is not less than 857 kg/m3, satisfactory low-temperature properties can be obtained. When the density is not more than 882 kg/m3, there is no fear that a lubricating oil composition is partially jellied at low temperatures owing to crystallization of an ethylene sequence portion in the ethylene/propylene copolymer. The density is measured in accordance with ASTM D1505-85. The molecular weight of the ethylene/propylene copolymer (A) , as measured by gel permeation chromatography, is in the range of 80,000 to 400,000, preferably 100,000 to 380,000, particularly preferably 120,000 to 350,000, in terms of a weight-average molecular weight of polystyrene. When the weight-average molecular weight is in the above range, the ethylene/propylene copolymer tends to have excellent properties in improving viscosity index. Therefore, a small amount of the ethylene/propylene copolymer suffices to obtain a specific lubricating oil viscosity, and the shear stability of the lubricating oil viscosity is high. When the molecular weight of the ethylene/propylene copolymer (A), as measured by GPC, is not less than 80,000 and less than 250,000, preferably 100,000 to 240,000, more preferably 120,000 to 240,000, in terms of a weight-average molecular weight of polystyrene, the ethylene/propylene copolymer tends to have excellent properties in improving viscosity index. Therefore, a small amount of the ethylene/propylene copolymer (A) suffices to obtain a specif ic lubricating oil viscosity, and the shear stability of the lubricating oil viscosity is high. Further, when the molecular weight of the ethylene/propylene copolymer (A) , as measured by GPC, is in the range of 250, 000 to 400, 000, preferably 260, 000 to 380, 000, more preferably 270, 000 to 350, 000, in terms of a weight-average molecular weight of polystyrene, the ethylene/propylene copolymer tends to have excellent properties in improving viscosity index. Therefore, a small amount of the ethylene/propylene copolymer (A) suffices to obtain a specific lubricating oil viscosity, and jellying hardly takes place at low temperatures. In the present invention, measurement of the weight-average molecular weight in terms of polystyrene by GPC is carried out under the conditions of a temperature of 140°C and a solvent of orthodichlorobenzene. The ethylene/propylene copolymer (A) has Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight), which is an indication of a molecular weight distribution, of not more than 2.3, preferably 1 to 2.2. If the molecular weight distribution is not more than 2.3, the shear stability of the lubricating oil viscosity becomes good when the copolymer is blended with a lubricating oil base. The melting point of the ethylene/propylene copolymer (A), as measured by DSC, is in the range of 15 to 60°C, preferably 25 to 50°C, more preferably 25 to 45°C. The melting point is a measure of an interaction between the ethylene/propylene copolymer and a pour point depressant. In order to prevent the interaction between the copolymer and the pour point depressant and not to inhibit the function of the pour point depressant, it is important that the quantity of the ethylene sequences having a melting point in the vicinity of -5 to +10°C contained in the copolymer is made as small as possible. The melting point is determined by obtaining an endothermic curve by means of a differential scanning calorimeter (DSC), and the temperature at the maximum peak position of the endothermic curve is taken as the melting point. More specif ically, a sample is placed in an aluminum pan, heated up to 200°C at a rate of 10°C/min, maintained at 200°C for 5 minutes, cooled to -150°C at a rate of 20°C/min and then heated at a rate of 10°C/min to obtain a 2nd run endothermic curve. From the obtained curve, the melting point is determined. The number of peak of the endothermic curve of DSC, which indicates a melting point of the ethylene/propylene copolymer (A), is preferably one. The density (D (kg/m3)) of the ethylene/propylene copolymer (A) andthemeltingpoint (Tm (°C) ) thereof asmeasured by a differential scanning calorimeter satisfy the following relation (I) Tm ≤ 1.247x0-1037 (I) , preferably the following relation (I-a) Tm ≤ 1.247x0-1039 (I-a) . The formula (I) and the formula (I-a) are each a measure of a composition distribution. When the density and the . melting point satisfy the above relation, the composition distribution of the ethylene/propylene copolymer is narrow, so that problems such as lowering of low-temperature properties of a lubricating oil caused by relative increase of the ethylene sequences having a melting point in the vicinity of -5 to +10°C and opaqueness (haze) of a lubricating oil caused by presence of a high ethylene content portion are not brought about. When the ethylene/propylene copolymer (A) has a weight-average molecular weight of not less than 80,000 and less than 250, 000, the ratio (n*0. 01/n*8) of its melt viscosity (n*0.0l) at 0.01 rad/sec, as measured at 190°C, to its melt viscosity (n*8) at 8 rad/sec, as measured at 190°C, is preferably in the range of 1.0 to 2.0. When the ethylene/propylene copolymer (A) has a weight-average molecular weight of 250,000 to 400, 000, the ratio (n*o . 01/T1*8) is preferably in the range of 1.5 to 2.5. The melt viscosity ratio mentioned above is a measure of long-chain branches contained in the ethylene/propylene copolymer, and a larger value of the melt viscosity ratio means a larger amount of long-chain branches contained in the copolymer. When the amount of long-chain branches in the ethylene/propylene copolymer is small, a lubricating oil composition containing the ethylene/propylene copolymer exhibits high shear stability of the lubricating oil viscosity. In the ethylene/propylene copolymer (A) of the invention, there is no specific limitation on the proportion (V (%) ) of o,p carbon atoms to all carbon atoms forming the copolymer and the ethylene content (E (% by weight)), but in a preferred embodiment, V (%) and E (% by weight) satisfy the following relation (IV) V > 10-0.1xE (IV). The ap carbon referred to herein is carbon which is secondary carbon in the main chain (or long branched chain) of the ethylene/propylene copolymer and, of the two tertiary carbon atoms nearest to which, one is carbon at the a-position (adjacent carbon in the main chain) and the other is carbon at the p-position (carbon adjacent to the a-positioned carbon in the main chain). The parameter V (proportion of ap carbon atoms) can be determined by measuring I^C-NMR of the copolymer and in accordance with the method described in J.C. Randall "Macromolecules" (11, 33 (1978)). The ethylene/propylene copolymer (A) having the above properties (a-1) to (a-5) may have the later-described relation (II) or (III) between the ethylene content and the melting point. The ethylene/propylene copolymer (A) (viscosity modifier for a lubricating oil) exhibits a high effect of improving viscosity index when blended with a lubricatingoilbase, hardly hinders the function of a pour point depressant and hardly brings about a problem of opaqueness of a lubricating oil. When the ethylene/propylene copolymer (A) is blended with a lubricating oil base, the resulting lubricating oil has excellent flowability at low temperatures and exhibits high shear stability of the lubricating oil viscosity. When the ethylene/propylene copolymer (A) is used as a viscosity modifier, a lubricating oil capable of satisfying low-temperature property standards of the GF-3 standards, which are standards of North America Lubricating Oil Standards of the next generation, can be obtained. Whether the lubricating oil satisfies the GF-3 standards or not can be judged by measuring the later-described CCS and MRV. The ethylene/propylene copolymer (A) can be obtained by copolymerizing ethylene, propylene, and if necessary, other monomers, in the presence of an olef in polymerization catalyst. Examples of the olefin polymerization catalysts employable in the preparation of the ethylene/propylene i, copolymer (A) include catalysts comprising a compound of a transition metal such as vanadium, zirconium or titanium and an organoaluminum compound (organoaluminum oxy-compound) and/or an ionizing ionic compound. Of these, preferably employed is: (a) a vanadium catalyst comprising a soluble vanadium compound and an organoaluminum compound, or (b) a metallocene catalyst comprising a metallocene compound of a transition metal selected from Group 4, etc. of the periodic table and an organoaluminum oxy-compound and/or an ionizing ionic compound. Of the above catalysts, the vanadium catalyst (a) is particularly preferably employed. These catalysts are described later. Another embodiment of the viscosity modifier for a lubricating oil according to the invention comprises the following ethylene/propylene copolymer (B). Ethylene/propylene copolymer (B) Theethylene/propylenecopolymer (B) comprises recurring units derived from ethylene and recurring units derived from propylene. The content of recurring units derived from ethylene (ethylene content) is in the range of usually 70 to 79 % by weight, preferably 71 to 78 % by weight, more preferably 72 to 78 % by weight, still more preferably 73 to 77 % by weight, particularly preferably 75 to 77 % by weight. The residue is a content of recurring units derived from propylene and the like. When the ethylene content is not less than 70 % by weight, satisfactory low-temperature properties can be obtained. When the ethylene content is not more than 79 % by weight, there is no fear that a lubricating oil composition is partially jellied at low temperatures owing to crystallization of an ethylene sequence portion in the ethylene/propylene copolymer. In the ethylene/propylene copolymer (B) , recurring units derived from at least one monomer selected from ct-olefins of 4 to 20 carbon atoms, cycloolefins, polyenes and aromatic olefins may be contained in amounts of, for example, not more than 5 % by weight, preferably not more than 1 % by weight, within limits not detrimental to the objects of the invention. The molecular weight of the ethylene/propylene copolymer (B) , as measured by GPC, is not less than 80, 000 and less than 250,000, preferably 100,000 to 240,000, particularly preferably 120,000 to 240, 000, of a weight-average molecular weight in terms of polystyrene. When the weight-average molecular weight is in the above range, the ethylene/propylene copolymer tends to have excellent viscosity index improvability. Therefore, a small amount of the ethylene/propylene copolymer suffices to obtain a specific lubricating oil viscosity, and the shear stability of the lubricating oil viscosity is high. The ethylene/propylene copolymer (B) has Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight), which is an indication of a molecular weight distribution, of not more than 2.3, preferably 1 to 2.2. If themolecularweight distribution is in the above range, the shear stability of the lubricating oil viscosity becomes good when the copolymer is blended with a lubricating oil base. The melting point of the ethylene/propylene copolymer (B) , as measured by a differential scanning calorimeter (DSC), is in the range of 15 to 60°C, preferably 25 to 50°C, more preferably 25 to 45°C. The melting point is a measure of an interaction between the ethylene/propylene copolymer and a pour point depressant. In order to prevent the interaction between the copolymer and the pour point depressant and not to inhibit the function of the pour point depressant, it is important that the quantity of the ethylene sequences having a melting point in the vicinity of -5 to +10°C contained in the copolymer is made as small as possible. The number of peak of the endothermic curve of DSC, which indicates a melting point of the ethylene/propylene copolymer (B), is preferably one. The ethylene content (E (% by weight)) in the ethylene/propylene copolymer (B) and the melting point (Tm (°O) of the copolymer (B) as measured by DSC satisfy the following relation (II) 3.44xE-206 ≥ Tm (II) , preferably the following relation (II-a) 3.44xE-208 ≥ Tm (II-a). The formula (II) and the formula (II-a) are each a measure of a composition distribution. When the ethylene content and the melting point satisfy the above relation (II), the composition distribution of the ethylene/propylene copolymer is narrow, so that problems such as lowering of low-temperature properties of a lubricating oil caused by relative increase of the ethylene sequences having a melting point in the vicinity of-5 to+10°C and opaqueness (haze) of a lubricating oil caused by presence of a high ethylene content portion are not brought about. The ratio (r|*o. 01/^1*8) of a melt viscosity (TI*Q.OI) of the ethylene/propylene copolymer (B) at 0. 01 rad/sec to a melt viscosity (Ti*g) thereof at 8 rad/sec, as measured at 190°C, is preferably in the range of 1.0 to 2.0. The melt viscosity ratio mentioned above is a measure of long-chain branches contained in the ethylene/propylene copolymer, and a larger value of the melt viscosity ratio means a larger amount of long-chain branches contained in the copolymer. When the amount of long-chain branches in the ethylene/propylene copolymer is small, a lubricating oil composition containing the ethylene/propylene copolymer exhibits high shear stability of the lubricating oil viscosity. In the ethylene/propylene copolymer (B) of the invention, there is no specific limitation on the proportion (V (%)) of o,p carbon atoms to all carbon atoms forming the copolymer and the ethylene content (E (% by weight)), but in a preferred embodiment, V (%) and E (% by weight) satisfy the following relation (IV) V > 10-0.Ix(IV). The ethylene/propylene copolymer (B) having the above properties (b-1) to (b-5) may have the aforesaid relation (I) between the density and the melting point. Theethylene/propylenecopolymer (B) (viscositymodifier for a lubricating oil) exhibits a high effect of improving viscosity index when blended with a lubricating oil base, hardly hinders the function of a pour point depressant and hardly brings about a problem of opaqueness of a lubricating oil. When the ethylene/propylene copolymer (B) is blended with a lubricating oil base, the resulting lubricating oil has excellent flowability at low temperatures and exhibits high shear stability of the lubricating oil viscosity. When the ethylene/propylene copolymer (B) is used as a viscosity modifier, a lubricating oil capable of satisfying low-temperature property standards of the GF-3 standards, which are standards of North America Lubricating Oil Standards of the next generation, can be obtained. Whether the lubricating oil satisfies the GF-3 standards or not can be judged by measuring the later-described CCS and MRV. The ethylene/propylene copolymer (B) can be obtained by copolymerizing ethylene, propylene, and if necessary, other monomers, in the presence of an olef inpolymerization catalyst. Examples of the olefin polymerization catalysts employable in the preparation of the ethylene/propylene copolymer (B) include catalysts comprising a compound of a transition metal such as vanadium, zirconium or titanium and an organoaluminum compound (organoaluminum oxy-compound) and/or an ionizing ionic compound. Of these, preferably employed is: (a) a vanadium catalyst comprising a soluble vanadium compound and an organoaluminum compound, or (b) a metallocene catalyst comprising a metallocene compound of a transition metal selected from Group 4, etc. of the periodic table and an organoaluminum oxy-compound and/or an ionizing ionic compound. Of the above catalysts, the vanadium catalyst (a) is particularly preferably employed. These catalysts are described later. A further embodiment of the viscosity modifier for a lubricating oil according to the invention comprises the following ethylene/propylene copolymer (C). Ethylene/propylene copolymer (C) The ethylene/propylene copolymer (C) comprises recurring units derived from ethylene and recurring units derived from propylene. The ethylene content is in the range of usually 70 to 79 % by weight, preferably 71 to 78 % by weight, more preferably 72 to 78 % by weight, still more preferably 73 to 77 % by weight, particularly preferably 75 to 77 % by weight. The residue is a content of recurring units derived from propylene and recurring units derived from other monomers described later. When the ethylene content is not less than 70 % by weight, satisfactory low-temperature properties can be obtained. When the ethylene content is not more than 79 % by weight, there is no fear that a lubricating oil composition is partially jellied at low temperatures owing to crystallization of an ethylene sequence portion in the ethylene/propylene copolymer. In the ethylene/propylene copolymer (C) , recurring units derived from at least one monomer selected from a-olefins of 4 to 20 carbon atoms, cycloolefins, polyenes and aromatic olefins may be contained in amounts of, for example, not more than 5 % by weight, preferably not more than 1 % by weight, within limits not detrimental to the objects of the invention. The molecular weight of the ethylene/propylene copolymer (C) , as measured by GPC, is in the range of 250, 000 to 400, 000, preferably 260,000 to 380,000, more preferably 270,000 to 350,000, of a weight-average molecular weight in terms of polystyrene. When the weight-average molecular weight is in the above range, the ethylene/propylene copolymer tends to have excellent viscosity index improvability. Therefore, a small amount of the ethylene/propylene copolymer suffices to obtain a specific lubricating oil viscosity, and jelly ing hardly takes place at low temperatures. The ethylene/propylene copolymer (C) has Mw/Mn (Mw: weight-average molecular weight, Mn: number-average molecular weight), which is an indication of a molecular weight distribution, of not more than 2.3, preferably I to 2.2. If themolecular weight distribution is in the above range, the shear stability of the lubricating oil viscosity becomes good when the copolymer is blended with a lubricating oil base. The melting point of the ethylene/propylene copolymer (C) , as measured by DSC, is in the range of 15 to 60°C, preferably 25 to 50°C, more preferably 25 to 45°C. The melting point is a measure of an interaction between the ethylene/propylene copolymer and a pour point depressant. In order to prevent the interaction between the copolymer and the pour point depressant and not to inhibit the function of the pour point depressant, it is important that the quantity of the ethylene sequences having a melting point in the vicinity of -5 to +10°C contained in the copolymer is made as small as possible. The number of peak of the endothermic curve of DSC, which indicates a melting point of the ethylene/propylene copolymer (C), is preferably one. The ethylene content (E (% by weight)) in the ethylene/propylene copolymer (C) and the melting point (Tm (°C)) of the copolymer (C) as measured by DSC satisfy the following relation (III) 3.44xE-204 ≥ Tm(III), preferably the following relation (III-a) 3.44xE-206 ≥ Tm (III-a) . The formula (III) and the formula (III-a) are eachameasure of a composition distribution. When the ethylene content and the melting point satisfy the above relation (III), the composition distribution of the ethylene/propylene copolymer is narrow, so that problems such as lowering of low-temperature properties of a lubricating oil caused by relative increase of the ethylene sequences having a melting point in the vicinity of -5 to +10°C and opaqueness (haze) of a lubricating oil caused by presence of a high ethylene content portion are not brought about. The ratio (n* .01/n*8) of a melt viscosity (n*o.0l) of the ethylene/propylene copolymer (C) at 0. 01 rad/sec to a melt viscosity (n*8) thereof at 8 rad/sec, as measured at 190°C, is preferably in the range of 1.5 to 2.5. The melt viscosity ratio mentioned above is a measure of long-chain branches contained in the ethylene/propylene copolymer, and a larger value of the melt viscosity ratio means a larger amount of long-chain branches contained in the copolymer. When the amount of long-chain branches in the ethylene/propylene copolymer is small, a lubricating oil composition containing the ethylene/propylene copolymer exhibits high shear stability of the lubricating oil viscosity. In the ethylene/propylene copolymer (C) of the invention, there is no specific limitation on the proportion (V (%) ) of ap carbon atoms to all carbon atoms forming the copolymer and the ethylene content (E (% by weight)), but in a preferred embodiment, V (%) and E (% by weight) satisfy the following relation (IV) V > 10-0.IxE (IV). The ethylene/propylene copolymer (C) having the above properties (c-1) to (c-5) may have the aforesaid relation (I) between the density and the melting point. The ethylene/propylene copolymer (C) (viscositymodifier for a lubricating oil) exhibits a high effect of improving viscosity index when blended with a lubricating oil base, hardly hinders the function of a pour point depressant and hardly brings about a problem of opaqueness of a lubricating oil. When the ethylene/propylene copolymer (C) is blended with a lubricating oil base, the resulting lubricating oil has excellent flowability at low temperatures and exhibits high shear stability of the lubricating oil viscosity. When the ethylene/propylene copolymer (C) is used as a viscosity modifier, a lubricating oil capable of satisfying low-temperature property standards of the GF-3 standards, which are standards of North America Lubricating Oil Standards of the next generation, can be obtained. Whether the lubricating oil satisfies the GF-3 standards or not can be judged by measuring the later-described CCS and MRV. The ethylene/propylene copolymer (C) can be obtained by copolymerizing ethylene, propylene, and if necessary, other monomers, in the presence of an olef in polymerization catalyst. Examples of the olefin polymerization catalysts employable in the preparation of the ethylene/propylene copolymer (C) include catalysts comprising a compound of a transition metal such as vanadium, zirconium or titanium and an organoaluminum compound (organoaluminum oxy-compound) and/or an ionizing ionic compound. Of these, preferably employed is: (a) a vanadium catalyst comprising a soluble vanadium compound and an organoaluminum compound, or (b) a metallocene catalyst comprising a metallocene compound of a transition metal selected from Group 4, etc. of the periodic table and an organoaluminumoxy-compound and/or an ionizing ionic compound. Of the above catalysts, the vanadium catalyst (a) is particularly preferably employed. Olefin polymerization catalyst The olefin polymerization catalyst used in the preparation of the ethylene/propylene copolymer (A), (B) or (C) is described below. In the preparation of the ethylene/propylene copolymer (A), preferably used is: (a) a vanadium catalyst comprising a soluble vanadium compound and an organoaluminum compound, or (b) a metallocene. catalyst comprising a metallocene compound of a transition metal selected from Group 4, etc. of the periodic table and an organoaluminum oxy-compound and/or an ionizing ionic compound; more preferably used is: (a-1) a vanadium catalyst comprising a soluble vanadium compound (v-1) and an organoaluminum compound; and particularly preferably used is: (a-2) a vanadium catalyst comprising a soluble vanadium compound (v-2) and an organoaluminum compound. In the preparation of the ethylene/propylene copolymer (B), preferably used is: (a) a vanadium catalyst comprising a soluble vanadium compound and an organoaluminum compound, or (b) a metallocene catalyst comprising a metallocene compound of a transition metal selected from Group 4, etc. of the periodic table and an organoaluminum oxy-compound and/or an ionizing ionic compound; and particularly preferably used is: (a-2) a vanadium catalyst comprising a soluble vanadium compound (v-2) and an organoaluminum compound. In the preparation of the ethylene/propylene copolymer (C), preferably used is: (a) a vanadium catalyst comprising a soluble vanadium compound (v-1) and an organoaluminum compound, or (b) a metallocene catalyst comprising a metallocene compound of a transition metal selected from Group 4, etc. of the periodic table and an organoaluminum oxy-compound and/or an ionizing ionic compound; more preferably used is: (a-1) a vanadium catalyst comprising a soluble vanadium compound (v-1) and an organoaluminum compound; and particularly preferably used is: (a-2) a vanadium catalyst comprising a soluble vanadium compound (v-2) and an organoaluminum compound. Soluble vanadium compound (v-1) The soluble vanadium compound (v-1) for forming the vanadium catalyst (a-1) that is preferably used in the preparation of the ethylene/propylene copolymer (A) or (C) is represented by the following formula. VO(OR)aXb or V(OR)cXd In the above formulas, R is a hydrocarbon group, such as an alkyl group, a cycloalkyl group or an aryl group; X is a halogen atom; and a, b, c and d are numbers satisfying the conditions of 0≤a≤3, 0≤b≤3, 2≤a+b≤3, 0≤c≤4, 0≤d≤4 and 3≤c+d≤4. Examples of the soluble vanadium compounds (v-1) represented by the above formulas include VOCl3, VO(OCH3)Cl2, VO(OC2H5)Cl2, VO(OC2H5)l.5Cll.5 VO(OC2H5)2C1, VO(0-n-C3H7)Cl2, VO (O-iso-C3H7) Cl2, VO (0-n-C4Hg) Cl2, VO(0-iso-C4H9)2Cl, VO (0-sec-C4Hg) Cl2, VO (0-t-C4H9) Cl2, VO(OC2H5)3, VOBr2, VC14, VOC12, VO (0-n-C4H9) 3 and VOC13-20C8H170H. Of the soluble vanadium compounds (v-1) , the following soluble vanadium compound (v-2) is preferable. Soluble vanadium compound (v-2) The soluble vanadium compound (v-2) for forming the vanadium catalyst (a-2) that is preferably used in the preparation of the ethylene/propylene copolymer (A) , (B) or (C) is represented by the following formula. VO(OR)aXb or V(OR)cXd In the above formulas, R is a hydrocarbon group, such as an alkyl group, a cycloalkyl group or an aryl group; X is a halogen atom; and a, b, c and d are numbers satisfying the conditions of 0