Pelican Parts has a Tech article on the subject, you can read it here: http://www.pelicanparts.com/bmw/techarticles/JF-Tech/BMW_E30_3_Series_differential_oil_change.htm
Buy the Bentley Manual for your car. It will pay for itself the first time you use it. You can get it from Pelican Parts, BMP Design, Bavarian Autosport, among others. GR