FORM 2 THE PATENTS ACT, 1970 COMPLETE SPECIFICATION [See Section 10] "PROCESS FOR THE PREPARATION OF SYNTHETIC OIL By OLIGOMERISATION OF A MIXTURE OF C6C8 DLEFINS4 INDIAN PETROCHEMICALS CORPORATION LIMITED, a Government Company, of P. O. Petrochemicals, District Vadodara - 391 346, Gujarat, INDIA, The following specification particularly describes the nature of the invention and the manner in which it is to be performed. A PROCESS FOR THE PREPARATION OF SYNTHETIC OIL BY OLIGOMERISATION OF A MIXTURE OF C6-8 OLEFINS USING AN ANHYDROUS A1C13 BASED CATALYST SYSTEM Field of the invention The present invention relates to a process for the preparation of synthetic oil by oligomerization of a mixture of C6-8 olefins using an anhydrous A1C13 based catalyst system. More particularly, the present invention also relates to a process for the preparation of synthetic oil of intermediate viscosity suitable for niche formulations. The oligomeric mixture is optionally fractionated/hydrogenated to furnish the desired synthetic oil. Background of the invention Oligomers of several olefins, especially 1 - olefins are highly useful in preparing functional fluids. Lubricants, hydraulic fluids, transformer fluids and the like generally use formulations with other functional fluid components and additives. Each functional fluid product must conform to established viscosity and volatility specifications. These olefinic oligomer products are generally prepared by the cationic polymerisation using Friedel-Crafts catalysts. Prior art discloses several processes for the oligomerization of olefins. US Patent No. 3,769,363 teaches oligomerisation of olefins with carboxylic acid-promoted boron trifluoride catalyst while, US Patent No. 4,436,947 discloses a process for producing a mixture of 1-decene oligomers with a catalyst comprising boron trifluoride and a mixture of an alcohol, a polyol, and a ketone. US Patent No. 4,935, 570 typically discloses 1-decene oligomerisation in the presence of BF3 along with promoters eg., water or alcohol. US Patent No. 3,842,134, on the other hand, teaches the use of AICI3-3CH3NO2 catalyst for the polymerisation of 1-decene to yield a high percentage of trimeric and tetrameric products. Prior art also discloses oligomerisation of olefins in the presence of aluminium halide alongwith HC1 (US Patent No. 3,637,503) The composition of the oligomeric mixture that is obtained from the oligomerisation reaction generally contains dimer, trimer, tetramer, pentamer and hexamer of the starting olefins such as hexene and octene. The presence of some amount of dimers of olefins affects inter alia the viscosity and the flash point specifications of the final formulations. The oligomer formulations produced are required to be normally of trimers and higher oligomers (>C18) such that synthetic oil formulations with the desired viscosity and flash point specifications can be obtained. To overcome this problem, fractionation needs to be done to eliminate lower oligomers ( C18). Another problem associated with prior art processes is that the catalyst systems are not suitable for olefinic mixtures of varying composition. In addition, the reaction time is inconveniently long with an average yield, which adds to the energy consumption and therefore, the cost of the process rendering these processes less commercially viable. Objects of the invention Accordingly it is an object of the invention is to provide a process for oligomerisation of olefins which avoids the disadvantages of the prior art. It an important object of the present invention to provide a process for oligomerisation of olefins which is energy efficient, with a short reaction time and high yields. It is yet another object of the present invention to provide a process for oligomerisation of olefins which minimises , to the tune of < 10 %, the dimer formation during the oligomerisation reaction. Another object of the present invention is to provide an oligomerization process which is suitable for olefinic mixture substrates containing olefins of varying compositions. A further object of the invention is to provide a process for the preparation of synthetic oil from a mixture of C6 to C8olefins. Another object of the invention is to provide a process for the preparation of synthetic oil with intermediate viscosity intended for niche formulations. A further object of the invention is to provide a process for the preparation of synthetic oil that has consistent physico-chemical properties notwithstanding any variations in the composition of the oils. Yet, another object of the invention is to provide hydrogenated synthetic oil useful as an additive in niche formulations. Summary of the invention The above and other objects of the present invention are achieved by the oligomerisation process of the present invention employing a unique catalyst system. It has been surprisingly found that when olefins are oligomerised in the presence of an AlCl3 catalyst in conjunction with a short-chain alkyl or arylalkyl halide cocatalyst, excellent yield of the final product containing essentially >C18 oligomers are obtained in a very short reaction time. The production of unwanted dimers is less than 10 %. Though the amount of catalyst or cocatalyst employed is not critical to the reaction, it has been found that the moisture content in the olefin mixture substrate is quite critical. For effective oligomerisation reaction, it is necessary that the said moisture level is lower than 200 ppm, preferably in the range of 30 to 40 ppm. The suitable moisture level may be achieved by any conventional means and will not pose any problem to a skilled artisan. Accordingly the present invention provides a process for the preparation of synthetic oil, which comprises subjecting a mixture of C6-C8 olefins substrate to oligomerisation, wherein the moisture content said C6-C8 olefins substrate is less than 200 ppm and said oligomerisation is carried out in the presence of a catalyst system comprising anhydrous AICI3 and an optional co- catalyst selected from alkyl or arylalkyl halides of the kind such as herein described.. In one embodiment of the invention, the olefinic mixture comprises of C6-C8 olefins in varying proportions. In a further embodiment of the invention, the olefinic mixture comprises 1-hexene, 1-ethyl-1-butene, 2 - ethyl - 1 -butene, 3-methyl-1-pentene, hexadiene, 1,5-hexadiene and 1-octene. In a further embodiment of the invention, the mole ratio of the olefinic mixture to the catalyst system is from 100:1 to 40:1. In another embodiment of the invention, the co-catalyst is selected from the group comprising isopropyl halide, iso-butyl halide and t-butyl halide. In another embodiment of the invention, the oligomeric mixture obtained form the oligomerization reaction comprises a mixture of dimer, trimer, tetramer, pentamer, and hexamer of the starting olefins. In a further embodiment of the invention, the mole ratio of the catalyst to the co-catalyst is from 4:1 to 2:1. In another embodiment of the invention, the oligomerization step is carried out at a temperature ranging between 90 - 150°C, preferably 90 - 120°C. In a further embodiment of the invention, the process time ranges between 3 to 10 hours, preferably about 3 to 5 hours. The present invention also relates to a process for the preparation of stable synthetic oil from an olefinic mixture of C6-C8 olefins by subjecting the said olefinic mixture to oligomerization in the presence of anhydrous AICI3 catalyst system and an optional co-catalyst selected from alkyl or arylalkylhalides to obtain an oligomeric mixture, hydrogenating or fractionating said oligomeric mixture to obtain hydrogenated stable synthetic oil. The hydrogenation is preferably carried out in the presence of a supported metal catalyst of the kind herein described. In one embodiment of the invention, the process of hydrogenation is carried out at hydrogen pressures in a range between 60 to 100 kg/cm2. In another embodiment of the invention, the hydrogenation catalyst used is supported M/y-AI2O3, wherein M = Nickel or Palladium. In a further embodiment of the invention, the support for the hydrogenation catalyst is selected from kieselghar or alumina. Detailed description of the invention Ohgomerization of the olefin mixture results in an oligomeric mixture containing substantially dimers, trimers, tetramers, pentamers and hexamers of the starting olefins. The oligomer formulations produced are required to be normally of trimers and higher oligomers (>C18) such that synthetic oil formulations with the desired viscosity and flash point specifications can be obtained. However, ohgomerization of olefins results in the formation of certain dimers of the starting olefins that adversely affect, inter alia, the flash point and the viscosity specifications of the desired synthetic oil formulations. To overcome this problem, fractionation needs to be done to eliminate lower oligomers (