The viscosity recommendations have changed from when my owner's manual was printed to the most recent list of "approved flluids" from MB. That list can be obtained from MBUSA (1-800-FOR-MERC) for free or a very small charge.
I have read on the Ritter/Easley site that Mobil 1 5W-30 is no longer a factory approved oil -- not sure why. Mobil 1 15W-50, 0W-40, 10W-40, etc ARE approved.
I used to use Mobil 1 5W-30 but have switched to 15W-50 year round.