API specification GL-4 is a standard set by the American Petroleum Institute (API) that defines the properties and performance of gear oils. The "GL" designation stands for "Gear Lubricant", a lubricant for gears. Oils meeting this standard are primarily intended for manual transmissions and some gearboxes operating in mild to moderate conditions. They contain additives for extreme pressures, so-called "EP additives", which form a protective film on the surface of the gear teeth and prevent wear under high loads. A key feature of GL-4 oils is the balanced amount of these additives, which provides sufficient protection, but is not aggressive to parts made of yellow metals such as brass or bronze. These materials are often used for synchro rings in manual transmissions, and using an oil with a higher concentration of EP additives could cause corrosion and subsequent deterioration of shift function.