Guess again, the Tcase uses ATF fluid and has since early 80's, not gear
oil. For rear axle you can use 75w90 or 80w90 conventional gear oil as
it will flow a bit better in cold weather that 75w140 or 85w140 (with
multigrade oil they check viscosity at fixed low temp and a fixed high
anf no where in between.) In front GM spec 80w90 unless it is below zero
a lot (then use 75w90) because the spider gears are always turning in
that axle so they what a little more protection/oil thickness at startup
and it is warmed somewhat by engine too.
Mobil makes SYN ATF fluid but unless you live were it gets well below
zero a lot, you are really not gaining anything. I would change the
fluids agin since you got the truck used so you know first had were
everything is "at"
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