I am in a similar situation as you. I prefer to use the short list of approved synthetic oils approved by MB.
My first choice would be Mobil 1 0W40 but I have not seen it at the retail stores. My next choice would be Castrol Syntek 5W50 which I can find at the retail stores.
As winter is approaching I will keep a look out for Mobil 1 0W40.
I would have considered Valvoline Synpower 5W40 but recall reading a thread that they were blacklisted by BMW.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles