Mareo, please don't misunderstand that what I put applies ONLY to transmissions. I have an 85 300ZX with 240,000 miles and the only thing that has been in the trans is ATF, that's the way it came from the factory.
Always use gear lube in a rear end, they need it, and any thing else will trash one in very short order. Again thicker isn't always better, 85w/90 is fine for the cliimite you live in, but if it regularly sees temp. above 100 then use 85w/140.
Most important of all what ever you use make sure that it meets the API GL4 spec. or better, as in GL4, GL5, or GL6. that way you can be assured that your diff. will have addiquite lubrication.