The hydrolytic stability of magnesium overbased products is significantly improved by the addition or inclusion of a mixture or reaction product of an alkylphenol resin with a mixture of an alkenyl succinic anhydride and the acid thereof, and an alkyl amine. These magnesium overbased products are useful additives to fuel oils to improve their corrosion-causing tendencies. In one non-limiting embodiment, the alkylphenol resin is nonylphenol resin and the succinic anhydride is a partially hydrolyzed succinic anhydride having from 14 to 26 carbon atoms, along with the acid thereof. The mixtures and reaction products also serve as asphaltene inhibitors in the fuel oils.