What Does GL5 Mean On Gear Oil?

There are different kinds of gear oil containers on the market, and they are all covered in numbers and abbreviations that may not immediately make sense to the uninitiated. In fact, some people might not even know what gear oil is in the first place. If you have any kind of related knowledge or if you ever shopped for gear oil, you've likely encountered abbreviations like GL4 and GL5. These aren't exactly super secret government codes. You can think of them as a rating that tells you what kind of performance the oil delivers and what conditions it's built for.

Gear Lubricant Level 5 (GL5) is an American Petroleum Institute (API) rating, indicating the oil is made to handle high-pressure, heavy-load conditions. This type of oil contains more extreme pressure (EP) additives to keep metal parts from grinding together under heavy loads. As such, GL5 gear oil is usually recommended for hypoid gears and heavy-duty gear systems. The important part to remember is that GL5 gear oil isn't actually a measure of quality, and isn't automatically better than GL4 (or other GLs). Instead, they are all made for different performance and gear systems, but unfortunately, also smell bad, unlike motor oil.

GL ratings scale from GL1 to GL5, and the higher the number, the tougher the conditions the oil is meant for. However, it's worth noting that GL2 and GL3 are considered obsolete ratings, and GL1 is nowadays mostly used only in older manual transmissions. 

The key difference between GL4 and GL5 gear oils comes down to how much EP additive they contain. Basically, EP additives are chemical compounds that add a protective layer on gear surfaces, guarding against adhesive wear and protecting the mechanical components when the lubricating oil layer thins out and can no longer keep the components safely apart.

Typically, GL5 gear oil has about double the EP additive content compared to GL4. Since it has less EP than GL5, GL4 gear oil is the go-to for manual gearboxes and transaxles with spur or helical gears that run at moderate speeds and carry ordinary loads. As such, it's often recommended for things like manual gearboxes in passenger cars and light trucks, as well as farm equipment with synchronizer-style transmissions. In the end, if you're not sure which gear oil to use, you should check the owner's manual and follow the recommendations there, or consult with someone more knowledgeable. Otherwise, just like with mixing transmission fluids, you risk damaging your car.