My buddy came to visit me in WI. He wanted an oil change as it was due on his GS. The HD shop had the oil grade he needed and was on the way, but wouldn't do the change b/c it was a BMW.
I was surprised because this was a HD, Buell and Ural dealer and they have always treated me well.
The owner is a great guy and following the Golden Rule (he who has the gold, rules) my buddy bought the oil and we did the change in my garage.
My thinking is that the techs don't know BMW so the shop can't guarantee the work? Not sure, but that is my guess.