Have been reading a while, but my first post here. I've only had my M3 for a little over a year, and oil topics are of interest.
I bought a used car and, first thing I did after getting the car home was to replace all the fluids. I went to Brisbane BMW for that, and I know that they are using Shell 10W-60, as that's what they put in my car. For my next change, I bought the oil (and other consumables) that I wanted and had Motorline BMW do the service.
Anyways, just thought I'd add that point to the discussion - the dealerships aren't all using Castrol here in Aus anymore.