No oil drain plug
WARNING: This page is about a different car, the 2010 BMW 650i, 2009 BMW 650i, and 2008 BMW 650i. However, it is still accessible from the selected car via links, so may be relevant.
Open plug (1).
Drain and dispose of differential oil.
Add rear differential oil up to lower edge of opening for screw plug (1); if necessary, refer to REAR AXLE - TECHNICAL DATA
Replace sealing ring or screw plug.
Tightening torque, see 33 11 3AZ in 33 11 REAR DIFFERENTIAL CASE WITH COVER (with sealing ring).
Tightening torque, see 33 11 9AZ in 33 11 REAR DIFFERENTIAL CASE WITH COVER (with O-ring).