the best car oil for high mileage...High-mileage diesel engines require the use of a special oil. According to Mobil Oil's website, a heavy grade 5W-40 oil is best for use in vehicles that hold a diesel motor with high mileage.According to the report, 5W-40 oils also improve engine function in diesels that contain exhaust gas recirculation (EGR) and after-treatment systems.