Hello,
I just replaced all of my differential and transfer case oils about two weeks ago.
For the differentials i went with the castrol saf xo 75w-90
For the transfer case I went with the OE BMW fluid
I filled both of the differentials all the way up to the fill hole because that's what it says in the procedure in the Rheingold software (the same software the dealership says). It's true that in some other documents it was posted as 600ml, but I preferred to listen to Rheingold procedure and fill it up to the neck. The same went for the transfer case as well.
I didn't notice any improvement in the differentials but my dying transfer case is now better with the fresh oil, the trembling that I had when taking off with wheels turned to the mas is now gone and am only left with the bad sound...maybe i can prolong it's life a little more.
Cheers