Heat and the motion of the engine break down the long chains into shorter chains and those chains are broken down even more. Old oil is better than no oil and will at least keep the engine from seizing up. But as the molecule chains get shorter, the metal parts are more likely to come in contact with each other causing more wear inside your engine. When automobile oil changes from the amber color it starts out with to the black color of old oil, that’s partly because it’s getting dirty, but it’s also undergoing chemical changes that prevent it from doing its job as well as it used to. If you change your oil yourself, save the old oil and take it to a service station or recycling center since used motor oil is very toxic.