NOVELTY - Improving oxidation-stability of a biodiesel-containing fuel composition involves combining a middle distillate fuel containing at least one biodiesel component (greater than 5 vol.%) with a synergistic mixture of a hydrocarbyl-substituted succinimide dispersant and hydrocarbyl compound (I) and its tautomers and enantiomers. The weight ratio of the hydrocarbyl-substituted succinimide dispersant to hydrocarbyl compound (I) is 2:1 to 10:1.    USE - For improving oxidation-stability of a biodiesel-containing fuel composition; and for operating an engine on a middle distillate fuel containing at least one biodiesel component (greater than 5 vol.%) (all claimed).    ADVANTAGE - The method effectively reduces or eliminates the need for expensive fuel components i.e. the middle distillate fuel is devoid of disalicylidene diaminoalkane compounds and metal deactivators, and the fuel containing hydrocarbyl-substituted succinimide dispersant and hydrocarbyl compound has a total insolubles amount of not greater than 2.5 mg/100 ml as determined by ASTM D-2274; and maintains the oxidation stability of biodiesel containing fuels.    DETAILED DESCRIPTION - Improving oxidation-stability of a biodiesel-containing fuel composition involves combining a middle distillate fuel containing at least one biodiesel component (greater than 5 vol.%) with a synergistic mixture of a hydrocarbyl-substituted succinimide dispersant and hydrocarbyl compound of formula (I) and its tautomers and enantiomers. The weight ratio of the hydrocarbyl-substituted succinimide dispersant to hydrocarbyl compound (I) is 2:1 to 10:1.    R2=hydrocarbyl group having a number average molecular weight of 100-5000.    An INDEPENDENT CLAIM is included for operating an engine on a middle distillate fuel containing at least one biodiesel component (greater than 5 vol.%) involving formulating the fuel with a synergistic mixture containing the hydrocarbyl-substituted succinimide dispersant and the compound (I) to provide an oxidation resistant fuel composition; and operating the engine on the fuel composition.