The amount of extreme-pressure additive added to the mixture is the primary distinction between GL-4 and GL-5 gear oils. The amount of the EP ingredient in GL-5 oil is nearly double that of GL-4 oil.
All gear oils contain some phosphorous or sulphur ingredient. As the gears rub against one another, this additive that coats them wears off. The gears are shielded from wear by this method. Due to the additive’s ability to corrode metals with a high concentration of yellow metal, GL-4 is frequently utilised in autos.
According to their intended application, the American Petroleum Institute establishes standards for automotive gear lubricants. The lubricant is designated as suitable for spiral-bevel and hypoid gears in vehicle axles by the API category GL-4 for gear lubricants. The vehicle should only be driven at reasonable loads and speeds. For a few specific manual transmission and transaxle applications, GL-4 oils are suitable.
The lubricant is designated as being suitable for hypoid gears in vehicle axles by the API category GL-5 for gear lubricants. For conditions including high speed, low speed, and high torque, the lubricant is adequate. Some oils are approved by the U.S. Military under the designations MIL-L-2105D, MIL-PRF-2105E, and SAE J2360, all of which comply with GL-5 gear oil specifications. But SAE J2360 gear oil performs better than GL-5 brands.