GL-4 or GL-5? What's the difference?

The primary difference between GL-4 and GL-5 lies in their intended applications. Typically GL-4 fluids are designed for use in some synchromesh manual transmissions, while GL-5 fluids are formulated for both manual and hypoid gear applications.

GL-4 and GL-5 are specifications set by the American Petroleum Institute (API) that determine the performance characteristics of the gear oil. Also unlike engine oils, the API specifications for gear oils are not backwards compatible with each other. An API GL-5 gear lube will not cover the API GL-4 specification. In addition, the test methods that were used to qualify a gear lubricant as meeting API GL-4 no longer exists.

One of the key factors to consider when choosing between GL-4 and GL-5 is heat and oxidation resistance. Manual transmissions typically generate less heat compared to hypoid gears found in differentials. Therefore, GL-4 fluids may not have the same level of heat resistance as GL-5 fluids.

It's well know that by dropping your fluid temperatures by 18 degrees you will double the life of the seals and the fluid. Schaeffer's transmission fluids and gear oils have a reputation for lowering operating temperatures.

GL-5 transmission fluids contain as much as 50% additional EP additives than GL-4. This will provide enhanced protection against extreme pressure, higher temperatures, and oxidation. These additives help prevent wear and tear on gears under heavy loads or high-speed conditions. It's important to note that using GL-5 rather than GL-4 fluid does not necessarily mean better performance!

Always refer to your vehicle manufacturer's recommendations regarding which specification is suitable for your specific transmission and drive system. Using the wrong fluid may damage your equipment!

In summary, if you have a manual transmission, using a GL-4 fluid will be sufficient. However, if your vehicle has a manual transmission with hypoid gears or if you're unsure about the type of gears in your vehicle, it's best to consult your owner's manual or seek professional advice to determine whether a GL-5 fluid is recommended.