It's my understanding that Mercedes now has two specifications for oil (gasoline engines). For engines manufactured from 1997 they recommend 0W-40 Mobil 1. There is a different specification for all others. Check the oil container to see if the oil meets the MB specification.
I do know that 0W-30, 5W-30 and 10W-30 DO NOT meet MB specifications (even if it's a synthetic).
I'd recommend 10W-40, 15W-50 or 20W-50 depending on where you live. Low viscosity oil has been pushed by auto companies to help them with their CAFE averages under the guise that it's good for the consumer. As someone once said, you have to wonder if any of those auto engineers in Detroit actually use that crap they recommend.
I change my own oil and use Mobil 1 15W-50. The cost of this oil and MB filter is no more expensive than if I took my car to some fast oil change place that uses no name oil and American filters.
BTW, look under the hood of a new Mercedes and notice the sticker that says you should use Mobil 1.