Abstract: A lubricating oil composition reduces the adverse effects of oxidation in a medium speed diesel engine. That lubricating oil composition has a major amount of a base oil of lubricating viscosity, from 1% to 30% of a highly overbased oil-soluble sulfurized alkaline earth metal hydrocarbyl phenate-carboxylate and from 0.1% to 5% of a polyalkylene succinimide prepared by reacting under reactive conditions a mixture of an alkenyl or alkylsuccinic acid derivative, an unsaturated acidic reagent copolymer of an unsaturated acidic reagent and an olefin, and a polyamine. When that lubricating oil composition has 0.1% to 2% of a physical mixture of from 20% to 90% of a zinc dialkyldithiophosphate derived from only primary alkyl alcohols, and from 10% to 80% of a zinc dialkyldithiophosphate derived from only secondary alkyl alcohols, the lubricating oil composition is useful for increasing the water tolerance of a medium speed diesel engine that are susceptible to water contamination.