A chemical mixture blended into motor fuels or motor oils to improve the performance of internal combustion engines.

For many years, tetraethyl lead was the primary gasoline additive. The introduction of this chemical into gasoline enhanced the octane rating and improved engine performance. When, for environmental reasons, the use of lead in gasoline began to be phased out, other chemicals were substituted.

Current additives include antioxidants, antistall agents, metal deactivators, corrosion inhibitors, deicing agents, and upper cylinder lubricants. These additives are usually blended into gasoline or motor oils, either at a refinery or terminal. They are derived from various exotic chemicals: amines, phenols, carboxylic acid, phosphates, etc.

See also Aromatics.