I've used 15-50 Mobil 1 in Porsches for years, and Castrol 20-50 in previous BMW's. I have had the oil changed in my 540 by the local dealer once at 1500 miles (no service light reset. They used the BMW stuff.
At 7500 mi now, and I'd like to change it myself. Mobil 1 doesn't seem to be avail in 5-40 wt, and I'm a little nervous about using the wrong oil.
BTW, I still have 4 grn lights in the service indicator bar. Car will be out of warranty before the first scheduled oil change, at this rate!