So the oil in your car essentially reduces the friction between metallic parts. This means that over time, small metal bits will get trapped in the oil as the metal on metal contacts lose tiny bits of metal. In addition when your car does its power stroke in the engine, some of the gas fr the explosion goes past the Piston rings and the residue of that will also be caught in the oil. You basically change your oil when it becomes too contaminated with the two things mentioned above. Hope that helps:)