Manual calls for a GL-4 75-90 yet most over the counter gear oils are
GL-5 "suitable for use in GL-4 applications" What brand and classification do you recommend? 3 quart capacity, correct?

 

I use 80-90w from the front wheels to the back wheels in my '92 Montero. The front & rear differential and the transfer case all get 80w-90.

As far as capacity goes, pump it in until it is spewing from the filler hole, then put the filler plug back on quick as possible.

 

Try CRC Sta-Lube...they have a GL-4 rated gear oil (it's 85W-90 which is still fine).