To obtain a lubricating oil having at least 120 viscosity index and ≤-45°C pour point by oligomerizing a mixed feedstock containing at least a heptene.

This lubricating oil is obtained by oligomerizing an olefin feedstock comprising a mixture of 7-9C olefins containing ≥30wt.%, preferably 40-75wt.% heptene in the mixture in the presence of an oligomerization catalyst comprising a halogenated alkylalminium and a halogenated alkyl (e.g. a hexane solution of ethylaluminium dichloride). An anhydrous ammonia gas is bubblingly introduced to the resultant reaction mixture so as to deactivate the catalysts. Further, water is added to the reaction mixture after deactivating the catalyst to wash the resultant reaction mixture and thereafter a solvent, etc., are evaporated from the washed mixture by a rotary evaporator to provide the lubricating oil. The molar ratio of the halogenated alkyl to the halogenated alkylaluminuim is within the range between 1.0 to 1 and 20 to 1 and the oligomerization is performed at ≤50°C temperature.