PURPOSE: To obtain a gear oil excellent in heat resistance an oxidiation resistance, by making a predetermined amt. of a specified carboxylic acid deriv. compsn. and a predetermined amt. of amine salt exist in the gear oil containing an additive supplement substance and forming an adherend by an L-60 test.

CONSTITUTION: One or more subsid. succinic acid acylating agent having a substituent originating from polyalkene having an Mn value of 500-1000 and a reactant selected from amine A having one or more primary or secondary amino group, alcohol (B), reactive metal (compd.) (C) and a combination of two or more kinds of components among A-C are reacted to obtain one or more carboxylic acid deriv. compsn. (e.g.; succinimide) and this compsn. and one or more amine salt of one or more long chain monocarboxylic acid and/or one or more long chain polycarboxylic acid (e.g.; octylamine salt of lauric acid) are allowed to be present in gear oil containing an additive supply substance containing one or more P-containing component, one or more S-containing component and one or more N-containing component in a predetermined ratio and forming an adhered of predetermined thickness on the surface of a gear by an L-60 test in an amt. of the adherend probally formed on the surface of the gear in the above mentioned test.