Replenishing rear differential oil:

WARNING: This page is about a different car, the 2002 BMW M Roadster, 2002 BMW M Coupe, 2001 BMW M Roadster, and 2001 BMW M Coupe. However, it is still accessible from the selected car via links, so may be relevant.

Undo oil filler plug (1).

Check rear differential oil level.

If necessary, pour in rear differential oil up to lower edge of opening for oil filler plug (1).

Fig 1: Identifying Oil Filler Plug

Courtesy of BMW OF NORTH AMERICA, INC.

Installation:

Oil filler plug with sealing ring:
Replace sealing ring (1).

Tightening torque 65 Nm.
Oil filler plug with O-ring:
Replace oil filler plug (2).

Tightening torque 60 Nm.

Fig 2: Identifying Oil Filler Plugs
Courtesy of BMW OF NORTH AMERICA, INC.