There was a guy here who used an ILSAC 0W-20 in his e36 328i during the winter in Canada.
I believe he was on Shannow's buddy list.
Since BMWs are not normally bought for their superior fuel efficiency, I'm not sure why you'd want to use a 0W-20 grade oil.
The reports of enhanced fuel economy using very light grades are greatly over-hyped. You could expect an improvement of maybe 1-2% as compared to a 0W-40 grade oil.
Although real, it would be barely noticeable in normal use. You might save .4-.8 gallons over each thousand miles.
I'd stick with the M1 0W-40 were it my BMW.