API stands for the American Petroleum Institute, which sets performance standards for lubricants. Its "GL" (Gear Lubricant) classification rates gear oils. The GL-4 standard is designed for manual transmissions and contains extreme pressure additives, called EP additives, which protect gears from wear. It marks an oil that exceeds the requirements of the GL-4 standard and provides better protection, but unlike oils of the higher GL-5 standard, it remains gentle on non-ferrous parts such as brass or bronze synchro rings.
