Abstract: A method of improving oxidation properties of biodiesel-containing fuel composition. The method provides combining a major amount of middle distillate fuel containing more than about 5 volume percent biodiesel components with a minor amount of a synergistic mixture of (A) a hydrocarbyl-substituted succinimide dispersant and (B) a compound of the formula: and tautomers and enantiomers thereof. In the formula, R2 is a hydrocarbyl group having a number average molecular weight ranging from 100 to 5000. The synergistic mixture has a weight ratio of A:B ranging from 2:1 to 10:1.