A 5W-20 gasoline engine oil which improves vehicle fuel efficiency by more than 1.5% relative to a conventional 5W-30 gasoline engine oil was newly developed. Its high fuel economy performance lasts 10,000 km. The viscosity was optimized to satisfy both fuel economy and antiwear performances. Thiadiazole was used to retain the initial fuel economy performance provided by MoDTC. 5 refs., 7 figs., 2 tabs.