A lubricating oil composition which comprises one or more base oils of the group consisting of (A) a mineral oil having a kinematic viscosity of 5-200 m.sup.2 /s at 40.degree. C. and an aromatics content of less than 15 percent by weight, (B) an olefinic polymer of 2-16 carbon atoms or its hydride having a number-average molecular weight of 250-4,000, and (C) an alkylbenzene having 1-4 alkyl groups of 1-40 carbon atoms and a total carbon number of said alkyl groups in the range of 6-40, said base oil or oils being blended with a 3-methyl-5-tert-butyl-4-hydroxyphenyl-substituted fatty acid ester in an amount of 0.1-5.0 percent by weight based on total composition. The inventive lubricant composition having such a base oil in combination with the specified fatty acid ester provides a synergistic effect conducive to inhibition or prevention of both oxidation and sludge formation while in use under elevated temperature conditions over extended periods of time.