Great posting and great information! This is such a volatile topic - everyone's got an opinion on which oil is best and is happy to share it!
I did a little research on the oils themselves - it seems that the ACEA A3 (2002) rating is the common thread behind all the highest performace-rated products from different oil suppliers. Castrol TWS and RS 10-60 have it, and so does Mobil 1 15-40. The ACEA rating for both companies' 5W-30 products is A1. The only 5w30 that I found (so far) that is rated A3 is the BMW-labeled product.
Maybe this is just a testing thing - perhaps BMW paid for the test to A3, and so they get the benefit of flogging the oil. Or maybe not... maybe there really is a difference...
More info on ACEA grading is available on
http://www.acea.be/ACEA/20020618Publ...lSequences.pdf
Cheers
JJ