Yes, you can put synthetic in your diffs, and that weight is fine. I run Amsoil 75w-90 in mine, as well as the swivel balls and everything has turned out great. One thing though, get a little extra just in case they leak, which they may.
I'm not 100% sure on the exact amount in each diff, but just keep pouring until you reach the top.
I'd say get 6 just to have extra.
Hope this helps.