Even with the best oil, after a while, the additives are depleted and the oil becomes too dirty to function effectively. Although improvements in engines and oils and the use of unleaded gasoline have extended “normal-service” oil-change intervals to as long as 20,000km for some vehicles, the recommended interval for “severe-service” conditions is three months or 5,000km, whichever comes first. Because most drivers operate under severe-service conditions, Ultra recommends a three-month/5,000km oil change interval. The time limit may come before the mileage limit. Your vehicle owner’s manual specifies the correct oil-change intervals for the vehicle.