An interim service is a simple oil change service, carried out at least every six months. A full service is a yearly check of engine oil, air filter, oil filter, etc. Mechanics recommend doing a full service once a year or more frequently if your car has high mileage. A major service has 20 or more items than a full service, usually carried out every two years. Expectedly, a major service costs more than a full service and an interim one.