AFAIK BMW doesn't make a 15W-40 synthetic. Their current oil for almost every BMW is a full synthetic 5W-30. The oil caps of the older cars say 15W-40, but that was a conventional oil.
I use the BMW 5W-30 in our Roadster, basically because with the BMW CCA discount, it is one of the cheapest full synthetics out there. I use Mobil 1 in two of our other cars (M3 LTW and Jeep Grand Cherokee). Castrol TWS 10W-60 in the E46 M3 because that is what is speced.
Most of our vehicles get the oil changed every year, as the mileage is not enough to trigger the change.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)