to be honest I really think oem is the best. I have never thought that there was any differance. i must admit that i am not compleletly knowlegdeable about these things. It is a six speed? or an auto?
I have to say that at the end of the day i dont really dont go in for spending heaps on performance **** because i drive it every day, every week, heaps of miles and oem spec seems to do quite well.
where ur at might make a differance cause if u have one of the last of the run models with all the goodies stacked on it would change the spec. 540i( but M) is a differant spec depending where u are)!
Try realoem.com to see whats the go
best of luck. hope some other fuka with a bit more knowledge can help u.