“Fuel additive” is a broad category of chemical treatments poured into the gas tank to improve engine performance or longevity. While it includes specific types like injector cleaners, general fuel additives often act as “conditioners.” They may contain lubricants to protect the fuel pump, corrosion inhibitors to prevent rust in metal tanks, or stabilizers to keep fuel fresh during storage. Some are designed specifically for diesel engines to prevent gelling in cold weather or to reduce soot emissions (DPF cleaners).