Having a mechanic do your oil change is a good idea if you do not have much experience with car engines. Oil can only hold so much dirt, and whether you do a lot of short trips around town or longer distance runs, eventually the accumulated dirt will lead to increased friction. By inventive use of different cleaners, penetrating oils, friction modifiers, oil stabilizers, and coating additives, the motor will chemically be "overhauled out of a can.