Question is do you have any original black, sulfur smelling type gear oil in the housing now?
Just cannot refill with store bought oil for fear of reaction. If present oil is identifyed, replace in kind. Otherwise draing old oil out, flush out, and refill with new favorite.
I've pulled the rearend out of the car for a rebuild. Yeah, I got plenty of the black stuff in there, but I'll have to replace the old black stuff with new black stuff and I'm trying to figure out the correct new black stuff to use.
Although the original black stuff costs less than modern brown stuff, it is now difficult to locate. Last time I found it was in an autoparts store in Texas.
Just go with the modern brown stuff, suitable for with or w/o locking diff.
I mixed in some of the brown stuff to the original black stuff, and was warned it did not mix (after it went in). Rear axle housing got quite warm going to work next day, so flushed and removed it all that night. Then ten years later there was yet another popular type of gear oil being used. It ahd big warning on contained not to mix. I think Sears used to sell it for about 10 cents less per quart. They no longer stock it, but was a warning.
Go with 90 weight full synthetics. It will do just fine. Synthetics hold up better than non-synthetics. The very best posi oil, which was never correctly synthesized, was used from the 1770s through the 1970s, was all natural and stunk like nasty dead crap. I believe it is no longer available in the U.S. Maybe Japan, I think they still kill whales.