Gear Oil Guide: 75W-90 vs 80W-90 and GL-4 vs GL-5 Explained

Gear oil works in some of the hardest conditions in a vehicle: heavy loads, high pressure and metal gears sliding against each other. The right gear oil keeps differentials, axles and manual transmissions quiet and protected. The wrong one can cause noise, poor shifting and early wear.

Reading the grade: 75W-90 vs 80W-90

Gear oil grades work like motor oil grades. The first number with the "W" shows how the oil flows when cold. The second number shows its thickness at operating temperature.

• 80W-90 is a common conventional grade for differentials and older equipment.
• 75W-90 flows better in cold weather and is often a synthetic. It is a good choice for Northeast winters.
• 75W-140 and 85W-140 are heavier grades used in some heavy-duty and towing applications.

GL-4 vs GL-5: what is the difference?

The API GL rating tells you how much extreme-pressure protection the oil has.

• GL-5 has more extreme-pressure additives. It is the standard for most hypoid gear differentials and rear axles.
• GL-4 has fewer of those additives. Many manual transmissions call for GL-4 because some older synchronizers made of brass or bronze can be harmed by GL-5 additives.

Always use the rating the vehicle or equipment maker specifies. Do not assume GL-5 is "better" for every job.

Conventional or synthetic?

Synthetic gear oil flows better when cold, resists breakdown at high temperatures and can extend change intervals where the maker allows it. Many fleets use synthetic gear oil in drive axles to reduce wear and improve fuel economy.

When to change gear oil

Check the service schedule for each vehicle. Change it sooner if the truck tows heavy loads, runs in severe service, or the axle has been under water. Inspect for leaks at the pinion seal and axle seals during every PM.

Gear oil at wholesale prices

RPS Distributors stocks gear oil from Mobil 1, Mobil Delvac, Chevron Delo, Super S, Starfire, CAM2 and Lubenet in quarts, gallons and bulk sizes. Browse our Lubricants collection.

Request a wholesale quote or call 1-855-498-7509.