Here's why I and my friends who drive BMWs that spec the 10w-60 are wary of alternatives:
- BMW's M division worked with Castrol to develop the engines and the oil together.
- BMW takes every opportunity to consolidate fluids across its models. They never use a different fluid for one of their cars unless it's necessary.
- We can be pretty sure this isn't a marketing thing to make the owners feel special, because the M division's turbo engines for the X5 M and X6 M do not specify the 10w-60. They run on the 5w-30 like almost every other BMW.
- Any time a manufacturer specifies something so unusual, and insists on it so adamantly for so long, it's worth listening.
- The Castrol 10w-60 in question may be one of the toughest street oils money can buy, period.
Now, this isn't to say there is definitely no alternative. Mobil insists their 0w-40 will do just fine, and given how cautious they are with what they say, I'm inclined to believe them. There are also a few alternatives in the thread below, although it is probably too soon to tell how those cars will hold up in the long run:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1570914
Bottom line: It's a gamble, and we don't even know enough to say how safe that gamble is. Unless you're particularly adventurous, I'd recommend sticking with BMW's recommended oil.