Abstract: A synthetic lubricating oil comprising an ?-olefin (co)polymer defined by the features (i) to (v). Also, a lubricating oil composition including the above-mentioned synthetic lubricating oil and a low-viscosity base oil. The synthetic lubricating oil and the lubricating oil composition have high thermal oxidation stability and are excellent in viscosity index, low-temperature viscosity properties and shear stability.

Abstract: A synthetic lubricating oil comprising an ?-olefin (co)polymer defined by the following features (i) to (v): (i) the (co)polymer contains constituent units derived from (a) at least one monomer selected from ?-olefins of 8 to 20 carbon atoms in amounts of 90 to 100% by mol and contains constituent units derived from (b) ethylene in amounts of 0 to 10% by mol, (ii) the dynamic viscosity at 100° C. is in the range of 8 to 500 mm2/s, (iii) the number-average molecular weight (Mn), as measured by gel permeation chromatography, is in the range of 500 to 15,000, (iv) the (co)polymer satisfies the following formulas (I) and (II): A?24.0×Exp (?0.0239×K) ??(I) B?0.0775×K ??(II) wherein A is a content (%) of a (co)polymer having a molecular weight, as measured by gel permeation chromatography, of not more than 1,000, B is a content (%) of a (co)polymer having a molecular weight of not less than 20,000, and K is a dynamic viscosity (mm2/s) at 100° C.