Lubricating oil composition which decreases copper corrosion and method of making same

Oil-soluble additive composition comprises an organic sulfur-containing extreme pressure additive, a thiadiazole, an amino phosphorus compound, and an alkyl or alkenyl succinic anhydride. The thiadiazole does not contain a polycarboxylate moiety. The amount of dimercaptothiadiazole in the thiadiazole is 0.1-10 wt.%. The alkyl or alkenyl group has a number average molecular weight of 160-700. Independent claims are also included for: (1) lubricating oil composition comprising a major amount of oil of lubricating viscosity, and a minor amount of the above oil-soluble additive composition; (2) method of making the oil-soluble additive composition comprising preparing a base package by mixing an organic sulfur-containing extreme pressure additive with an amino phosphorus compound, and mixing the resulting product with an alkyl or alkenyl succinic anhydride and a thiadiazole; and (3) method of making a lubricating oil composition comprising making the oil-soluble additive composition and combining the oil-soluble additive composition with a major amount of oil of lubricating viscosity.