The American Petroleum Institute (API) has established a set of ratings to better distinguish between the unique physical and performance characteristics of different types of Gear Lubricants. Gearboxes and transmissions at work in heavy duty vehicles are subject to varying degrees of pressures, temperatures, and demands depending on make, model, and various other factors. Likewise, gear lubricants have unique specifications of their own - this potentially renders one exceptionally well-performing gear lubricant in one application useless or harmful in another.
API GL-1 Gear Lubricants are marginally modified or unmodified mineral oils that are operating under very mild conditions, pressures, and velocities. These oils are intended for use in manual transmissions and may contain some oxidation and rust inhibitors, defoamers, and pour depressants but may not contain friction modifiers or extreme pressure (EP) additives. API GL-1 Gear Oils are also recommended for differentials, final drives, steering gears, universal joints, spline shafts, industrial closed gears and many other applications where a straight mineral gear oil is required.