PURPOSE: To produce an improved multi-quality synthetic hydrocarbon engine oil which is free from a viscosity increaser and can be used in both summer and winter by comprising a synthetic base composed mainly of a mixture of specified hydrogenated decene-1 oligomers (derivs.).

CONSTITUTION: This improved multi-quality synthetic hydrocarbon engine oil comprises a synthetic base comprising a mixture of hydrogenated decene-1 oligomers having a kinematic viscosity (100°C) of 7.1 to 12.5 cSt and an average mol.wt. of 550 to 798 with 19.8 to 24.7% of hydrogen atoms in the oligomer being hydrogen of methyl as a main component and not more than 21% of 30C oligomer, 5 to 64% of 40C oligomer, 17 to 45% of 50C oligomer, 6 to 35% of 60C oligomer, and 1 to 20% of at least 70C oligomer.