I know I'm a little late to this but I thought I'd chime in for the sake of correcting some fallacies presented here.
15,000 mile oil changes won't kill your car instantly, but they certainly decrease its service life. This is because of BMWs free maintenance combined with bragging rights that the car needs to be serviced so infrequently- if you plan on keeping your car however- you should change the oil more often.
3,000 miles for a normal interval (non break in) is too short for new oils, analysis has shown good synthetic oils to last between 7,000-10,000 miles based on wear reports and measure, 15,000 is not a safe bet. Additionally, the oil BMW uses is not actually a true synthetic, it is one of Castrol's highly refined "dino" oils, that passes lower synthetic testing standards.
The reason I bring this all up is because of the tremendous amount of carbon buildup E90 335 owners (and owners of other brand F/I direct injection cars) are reporting. This usually occurs in the CPO life cycle (50-100k), but BMW often denies the claims, for no good reason. The way to avoid this is to change your oil at a sane interval (~7500 miles) with a BMW LL-01 approved oil that is a true synthetic- which will have a higher flash point than BMW oil and will resist carbon buildup better. Long drain intervals are a poor idea with regards to carbon buildup because water content (IIRC) can increase with mileage leading to even poorer characteristics with regards to buildup (in the 7,500-15,000 mile window).
The two oils that come to mind for use are Mobil 1 0w-40 and Castrol Edge SPT (European Formula) 0w-30. They both will give your motor a better chance of resisting carbon buildup if you decided to stick with the ridiculous 15,000 mile interval, and it certainly doesn't hurt to use them every 7,500 miles either. I'd also recommend doing a first change in the M range (1200-2000 miles), but that is more a matter of opinion on these motors.