How do antioxidant additives extend the useful life of engine oil?

Antioxidant additives extend the useful life of engine oil by inhibiting the process of autoxidation, which is the chemical reaction between the oil and oxygen in the air.1

In engine environments, heat and metal catalysts can initiate a chain reaction involving free radicals—molecular fragments with unpaired electrons that act as chain carriers. This oxidative process leads to the degradation of the oil, resulting in the formation of undesirable substances like gums and sludge that compromise engine performance.1

Antioxidants, typically organic compounds such as aromatic amines or phenols, function by reacting with these free radicals to terminate the oxidative chain reaction. By donating hydrogen atoms to the chain carriers, these additives neutralize the reactive fragments, thereby preventing the oil from breaking down prematurely and maintaining its lubricating properties for a longer duration.1