Can 75W-90 Replace 75W-85 API GL-5 Gear Oil?

Updated April, 2024

A reader asks: I’ve got a 2012 Cadillac Escalade that calls for a 75W-85 API GL-5 gear oil in the rear differential. AMSOIL’ s application guide suggests Severe Gear 75W-90. Is this correct? Can I safely replace 75W-85 with 75W-90?

Our answer: Yes, 75W-85 gear oil can most definitely be replaced by 75W-90. In fact, we know that some GM dealers will use 75W-90 gear lube in applications where Chevrolet/Buick/GMC/Cadillac suggests 75W-85.

Why Do Car Makers Suggest 75W-85 GL-5 Gear Oil In Some Cars And Trucks?

We can only speculate on why car companies’ specify 75W-85 for some vehicles. Here are our theories.

 To boost fuel economy. Auto manufacturers have to meet government-mandated fuel economy standards across their product lines. If a car maker doesn’t meet these fuel economy targets, they can be subject to big fines. These fines can be in the tens of millions of dollars. That’s why automakers are going to lighter oils in the engines and transmissions and differentials.

You may not notice the fraction of a mile-per-gallon improvement because of the thinner oil in your diff. But the automaker bean counters do and they are banking on every trick in the book to boost collective fuel economy.

A 75W-85 GL-5 gear oil is relatively rare. Therefore, consumers will feel that the safe choice to go back to the dealer for service. There will be a natural fear to deviate from the exact viscosity in the owner’s manual. Again, we know for a fact that some car dealers simply use 75W-90 anyway.

Why Use AMSOIL Severe Gear 75W-90 Instead Of 75W-85?

1.  Very close viscosity. Most diffs were designed with 75W-90 in mind. Going to 75W-85 is a minor fuel-economy-boosting endeavor. AMSOIL Severe Gear 75W-90 is designed to replace, 75W-90, 75W-85, 80W-90, SAE 80 or SAE 90 gear oils. It can replace any of these with no worries or risk.
2. No fuel economy penalty. As AMSOIL Severe Gear 75W-90 is an ultra-low-friction synthetic formula. There will not be a measurable loss of fuel economy by going with this slightly thicker oil.
3. World-class wear control. Severe Gear 75W-90 is formulated to provide elite wear protection. It is designed to withstand high horsepower, heavy loads and extreme heat or cold.
4. Reasonable cost. At our wholesale prices, AMSOIL Severe Gear Synthetic 75W-90 is about half the cost of AC Delco 75W-85 API GL-5 Gear Oil (for example).

What About Limited-Slip Additives?

If your car or truck has a limited slip rear axle and requires the a friction modifier additive, definitely pick up a bottle of AMSOIL Slip-Lock Differential Additive. You an also use the Limited Slip Axle Lubricant Additive offered by your car dealership. AMSOIL Severe Gear contains limited-slip additive. But we find it’s still a good idea to add about 2/3 bottle of limited-slip additive (for good measure). If using another gear oil brand, add the full bottle of limited-slip additive.