Check to tag on the back of the rear end
if you have an LSD ( look for a S on the tag), then you'll need 140.
Use a good quality synthetic
Get new gaskets and use 75w90 Mobil1 synthetic.
As others have said, get the gaskets first and make sure you loosen the fill plug before draining out the axle :D
And, being entirely anal, I replace my tranny fluid (Mobil1 Synthetic ATF), rear axle fluid (Mobil1 75w90) and power steering fluid (Mobil1 Synthetic ATF) along with the brake fluid (Ate Typ200/Super Blue) and engine coolant (Genuine BMW w/Distilled water) every 2 years.