Oil oxidation occurs through the formation of acids within the engine. Sulfuric and Nitric acids are formed through the introduction of combustion byproducts and water. Water introduced through the combustion process and from the exposure to atmosphere, plays a critical part in the formation of acids that contribute to the reduced TBN (Total Base Number) of engine oil. As the acids accumulate, the oil additives intended to isolate acids within the oil are depleted. Once depleted, the additives can no longer prevent the breakdown of the oil.