PURPOSE: To produce a gear oil lubricant suitable to be used for a transmission equipped with a synchronizer.

CONSTITUTION: This lubricant has 4 to 32 mm2 sec-1 (cSt) kinematic viscosity at 100°C and contains (a) a base oil, (b) a gear oil additive package and (c) at least one kind of alkali and alkaline earth metal salts of oil-soluble sulfonic acids. The base oil contains at least 25 wt.% (i) hydrogenated poly-α-olefin oligomer oil or (ii) synthetic ester oil or (iii) combination of (i) and (ii). The gear oil additive package is used in such an amt. or larger that the gear oil lubricant satisfies the specifications concerning API-GL4 service. The alkali or alkaline earth metal salt of oil-soluble sulfonic acid (c) is used in an enough amt. to improve the frictional characteristics of the lubricant used for a manual transmission, especially a synchronizer-type manual transmission, and by 0.01 to 2 wt.% range based on the whole weight of the lubricant.