Heads up:
There is a theory that changing your oil more frequently can't hurt. This is true if you were to chose to change your oil yourself every 5000 or 7500 miles and then bring it to your dealer (or Mercedes specialist) and perform the proper A or B service. All you are doing is wasting money on Mobile 1 and expensive specialty Mercedes filters, and that can't hurt anything but your wallet. (It doesn't help one iota, but it doesn't hurt the car.)
However, if you use cheaper filters, cheaper oil, fail to go to the dealer every 12 months or 15,000 miles, or some combination thereof, you are hurting things. And your A service at ~$200 and B at ~$600 are expensive enough. They are as thorough and expensive as they are to accommodate the long service interval, both for the vehicle and the dealerships bottom line. Mercedes are unusual mechanically from the engine design to the lug bolts that secure the wheels.
Second, and more importantly, the Mercedes Metris does not have a 12 month/15,000 mile service interval. It has Mercedes Advanced Service SYSTem, or ASSYST, which allows for service intervals UP TO 12/15k. It tracks things like duty cycle, engine load, driving conditions, towing, environmental conditions, and so on. If you are using the system in a way that shortens the prudent service interval, the computer will note this and reduce the interval accordingly.
Mercedes offers this as a way to provide for better customer service and to make people bitch less about the extremely high servicing costs. It's actually cheaper to service than a Lexus- my mom pays $70 for an oil change service- every 5000 miles. But Mercedes does not want people unhappy with their cars- and Mercedes dying from metulurgical engine wear is practically unheard of (that's what oil changes prevent, by the by).
Fully synthetic oils like Mobil 1 don't actually break down. In fact, various tests suggest that if you frequently change the filter, the oil can last practically forever. The 12/15k service interval was set upon more on making sure the car gets inspected with adequate frequency than anything to do with oil life.
