I'm getting ready to change the differential and transfer case fluids in my AWD Lexus RX, and it calls for 75W-85 GL-5. This is hard to find and is pretty expensive if you can find it. Would it be OK to use 75W-90 since it is far more available and cheaper?
Yes.
Not recommended.
Toyota / Lexus chose to do with a certain fluid for a reason.
Besides, the correct fluid is $$ (at absolute worst maybe low $$$) while a new diff if your one gets damaged or worn out faster is low-mid $,$$$
seriously? 5 viscosity points?
Yeah, 5 viscosity points is like the difference between water and beer. @mmj
Stout and lager @doc
Both delicious!
Yeah. I'm not saying it's going to cause issues, what I am saying is that I would not recommend it.
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Manufacturers are very specific and have many different specs of mechanical gearbox (manual/diff/dry DCT/...) oils,
Very similar products in the same category require different oils - and that's just not an arbitrary choice.
It's also important to not forget about the GL-5 rating, as that means the oil can withstand the pressure conditions inside of the diff, as opposed to the regular GL-4 waiting for most modern manual transmissions.
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Is it probably going to be fine? Probably.
Is it a great idea to go against manufacturer oil viscosity specifications because of price/availability? No.
@mmj A 30% (according to the SAE standard, and test data) kinematic viscosity (a fluid's internal resistance to flow) difference at 65°-100°C is not that negligible.
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Higher viscosity means less flow, Is the Toyota diff built well enough to handle it? Most likely.
But a lot of diffs and manuals like Nissan-Renault inventions have issues with oil flow even with the recommended fluids.
Using 75w90 instead of 75w85 is not going to damage the differential.