here is what a quick google search turned up for you your mileage may vary and i'm certain that BMW has verified the 15000 mile interval as acceptable
from BMW Service Information Bulletin 11 06 01, October 2001
BMW has changed their engine-oil recommendation for all M cars equipped with the new S54 engine (E46 M3 coupe and convertible, 2001-on M coupe and M roadster) from the previous BMW High Performance Synthetic 5w30 engine oil (made by Castrol), pn 07 51 0 017 866 (1 quart), to Castrol TWS Motorsport 10W-60 Synthetic Engine Oil (previously known as Castrol Formula RS 10W-60. . . the oil is the same, only the name has changed) pn 07 51 0 009 420 (1 liter).
Presently, BMW recommends Castrol TWS Motorsport 10W-60 for all S54-equipped M-cars as well as all S62-equipped M5s and Z8s produced after 3/00, the factory recommends BMW High Performance Synthetic 5w30 engine oil. The oil change interval of approximately 15,000 miles or once a year remains unchanged.