bro, M54 engine is famous for burning oil, mine have the same problem, don't bother to overhaul / change oil separator, i have done that and i don't see much improvement except $ burned, i remember my friend got his new E60 (1st batch) in 2004, also running a M54 engine, his engine is burning oil too until now.
I have used 5w50 BHP fully syn and 10w50 total fully syn, seems to be the same, last week i just change to 10w40 total semi just to see if it burn less oil.
How do u find bardahl? i use it once, feels like cooking oil when touch/ maybe they sold me a snake oil. Anyway i opt for Total as it cost the same and it is definitely a better oil.