1960 Suburban Posi Rear Diff Fluid
 

As the title says, what weight fluid do I need to add to my rear diff? It's factory. I filled it a few years ago and added the posi fluid, but it looks like it has a small leak where it was cracked and welded up somewhere along the way. I plan to take the fill bolt out to check fluid levels and want to be prepared to add some back in if needed.

I have some BG Products 75w-90 and some 80w-90 (even have a quart or two of 80-90 marine, no clue how that differs)

Also, when is it a good idea to re-add the posi fluid? I think that's a lucas product. Or do I just top off with that?