In the manual it says you can use:
You might want to find out what (competent) BMW dealers use for the 1M, X5M, and X6M, since I believe they have the exact same oil spec as the M5. I'm reading that 5W-30 tends to be used in the US, but not sure about brand.
I'm not surprised that a dealer thought 10W-60 was needed, sadly. I just hope they'd notice the spec on the oil cap before actually putting that into someone's car....