Gear, or differential, oil is used to lubricate the gears in a car’s transmission so that it can shift smoothly and easily. This type of fluid is generally used on standard transmissions, whereas transmission fluid is used in automatic cars.
Differential oil is extremely high viscosity and can withstand the hot temperatures reached in the gearbox. Over time the level will drop to some extent, however, and you may need to top it off. If you notice grinding or difficulty when shifting gears, check the gear fluid. The gearbox is often located behind – and lower than – the engine, but check your owner’s manual to be sure. It may have just a plug, or there may be a dipstick as well. The oil should come up to the plug hole, allowing you to touch it. If it doesn’t, top it off until the fluid just starts to spill out of the hole.
When shopping for gear oil it’s important to understand both the API (American Petroleum Industry) and SAE (Society of Automotive Engineers) ratings. The API is expressed as GL-1, GL-2, etc. (GL stands for gear lube). This rating relates to the additives in the gear fluid designed to stop metal-on-metal contact between the gears.
SAE ratings are expressed in the same way as with engine oil, for example 75W-90, indicating the viscosity of the fluid. The higher the rating, the thicker it is.
Passenger vehicles typically take GL-4 gear fluid, but check your manufacturer’s recommendations before putting anything into your gear box.
How to make sure you’re getting good quality differential/gear oil
Consider a more expensive brand. Differential fluids like Amsoil and Red Line are a bit more expensive than those you’ll find in a big box store but they’ll need changing less frequently.
Don’t mix ratings of gear oil. Because of the different additives in the various types, they may not play nice together. Always drain the system first if you’re going to change types.
Be aware that differential fluid labeled as GL-4/GL-5 is really GL-5. If your car calls only for GL-4, do not use these “multipurpose” oils.
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This article originally appeared on YourMechanic.com as How to Buy Good Quality Differential/Gear Oil and was authored by Valerie Johnston.