Might not be the oil separator or could be a combo of things. Here's some threads with pics:

http://www.xoutpost.com/bmw-sav-foru...iller-cap.html

http://www.xoutpost.com/bmw-sav-foru...-pictures.html

See post #13 here:
http://www.xoutpost.com/bmw-sav-foru...perator-2.html

http://www.xoutpost.com/bmw-sav-foru...under-cpo.html