Does anyone have a link to Mobil 1's site or other documentation that shows using Mobil 1 forces you to always use Mobil 1?
I first tried it because it's easy to get.
The nice thing about synthetic oil is that you don't have to worry about tempertature as much - since it doesn't change like dinosaur oil does, just run the right weight for your engine, and be done with it. As mentioned above, 0-40 is thicker oil than 10-30, it's just much much more consistant across a variety of temperatures.
For my M42 I ran M1 5-30 which I got analysis done every time I changed the oil. It showed this oil worked great for what I did with the engine, that is approximately 40% highway, 60% on the track. Under these conditions oil analysis proved that this oil would let that engine run safely fror about 7500 miles between changes. This doesn't necessarily mean that M1 5-30 is perfect for all conditions, just good for what I was using it for.
For my new M52 I've gone with Mobil 1's recommendation of 0-40. When I change that, I'll get it analyzed and see if it's working properly. If not, I'll change to something else.
Bottom line,
1. Go with some oil you've researched as good for the car
2.
Get it Analyzed.
3. Adjust viscosity / use cycle based on the analysis.
Neither an oil company nor BMW can tell you what oil is perfect for your car based on the way you drive it. Find out for yourself.