German Castrol 0W-30. Seriously, it is some of the best high performance oil that you can buy for turbo cars. It compares well to Redline, Motul etc and meets BMW certifications. Plus it is readily available in parts stores. I used it in my STI and currently use it in my 335. Oil analysis after 10k miles is great and indicates that I can go longer if I want.
No oil is going to give you an edge if you are already using a high quality synthetic. Just pick one and stick with it.