Oil sludge is the breakdown product of over-stressed oil in your engine. The oil that is stressed by contaminants and oxidation will break down into a gel that sticks to your engine parts. As the sludge sticks, there is less good oil to circulate and do its protective job. This coating of gel also stores heat instead of releasing it which stresses the radiator and cooling system. A sludgy engine is being damaged with EVERY stroke. Your engine may lose oil pressure, get terrible gas mileage, and other components might mysteriously fail such as timing belts, idle speed controls, and gaskets.