80w90 is not ok in a rear end that requires 75w140 synthetic and if you have a limited slip rear end than you will also need the proper amount and the proper type of friction modifier
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Not really. 80w90 is the almost original rear end catch all/cover all fluid.
75w140 is the broader coverage and IIRC always at least a syn mix to get this large spread in numbers.
Some trucks sold with 80w90 and then the manufacturer later superseded to go with 75w-140.
75w140 on paper should give better protection but there have bee tests that show increased heat with it vs 80w90.
You could say you can put 75w140 in even ones that call for 80w90 but you can put 80w90 in everything just the same and not going to lose any rear ends or have any shorter life in the real world. It's not really about what kind of gear lube you have in it...it's about having enough and it not being contaminated with water.
