Ran a mixed municipal fleet of 1,200 units. Always pumped (bulk) Valvoline All Fleet 15w40. Used this oil in diesel ambulances, Crown Vic Police cars, standard admin compact cars, and off road equipment. Ambulance and Police car service intervals were based on rough service/severe duty schedules. Remaining fleet was based on 6,500 miles intervals. Off road equipment service was based on manufacturer intervals (Hours). Used NAPA-Wix oil and air filters. Never any oil related problems.