In the present work, impact of biodiesel on engine oil quality was focused on the roles of methyl oleate and the performance characteristics of an engine oil metallic sulfonate detergent additive. Oxidation and detergency tests of blends of methyl oleate, over-based calcium alkylarylsulfonate and n-hexadecane or HVI 350 mineral base oil were performed on an accelerated oxidation tester, a PDSC, and a panel-coking simulator, respectively. Thereafter the viscosity and acidity of fresh and oxidized samples were determined and the chemical compositions analyzed by GC/MS. Results showed that methyl oleate greatly promoted oxidation of n-hexadecane and the mineral base oil, typically characterized by markedly reduced oxidation stability, sharply increased viscosity and acidity, and drastically decreased detergency. However, increase in viscosity and acidity and decrease in detergency caused by methyl oleate could be well retarded by calcium sulfonate. Moreover, direct chemical interactions between methyl oleate and calcium sulfonate seemed unnoticeable. The present results evidently demonstrated that methyl oleate played significant roles in accelerated oxidation degradation of n-hexadecane and the mineral base oil, and that calcium sulfonate had worked properly and efficiently with excellent performances in improving detergency and inhibiting increases of acidity and viscosity of oils contaminated by methyl oleate.