Extreme heat hastens chemical breakdown (oxidation), altering the oil’s chemistry and preventing it from lubricating, cooling and protecting as designed. Sludge can form, which clogs narrow oil passages and prevents oil from reaching vital components, causing wear. Oxidation can also lead to increased oil viscosity, which negatively affects energy efficiency. Ultimately, the oil loses its effectiveness and must be changed.Contaminant build-up also affects oil life. Motor oil is formulated with detergent and dispersant additives responsible for holding dirt, soot (in diesel engines) and other contaminants in suspension and preventing them from depositing on engine surfaces or forming sludge. These additives are sacrificial in nature, meaning once they’re depleted, the oil’s effectiveness suffers and it must be changed.