I recommend using Mobil 1 15w50.
I also recommend that you do an "in between" oil change at about 6000 miles.The FFS will have you change it about every 12,000,but I've seen some pretty ugly low mileage (45000)
engines that were serviced by FSS intervals.
You should also rotate tires about every 5000miles.
If it has'nt been done by the first "A" service,Its probably too late.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."