How often do you change the air in your tires? How about your brake and radiator fluids? You may need to fill them up or change them once in a while, but you don’t have to change them the way you have to change your oil. You should change the water in your radiator because it gets dirty, and that’s one of the reasons to change your oil as well. But there’s another, more significant reason that old oil doesn’t work as well as new oil. Inside your car’s engine, metal parts are constantly sliding across each other at extremely high speeds. The friction of metal against metal at that speed could generate enough heat to destroy the engine — if there weren’t oil to lubricate the metal parts and help them slide more easily. The long molecules of oil work their way between the metal parts so that moving metal parts never really touch each other. But those long chains don’t last forever.