You did not specify which Mobil 1 formula you have been using. While there are a couple Mobil 1 and Mobil-Delvac 1 formulas that DO meet the MB specs,
MOST DO NOT .
Chevron Delo 400 15W-40, Mobil Delvac 1300 Super 15W-40 and Shell Rotella T 15W-40 are all formulas that meet MB specs for your Sprinter and are all highly regarded by large commercial fleet operators. They are all available at around the same price ($9.89/gal at our local WalMart).
If you have a couple hours to spend, try this site
http://www.bobistheoilguy.com/ . My guess is that if you do you will settle on the Rotella T (or one of the other two mentioned above) (the site has some specific topics on Syn vs. Dino as well as the best oil for the MB 5 cylinder 2.7 liter Turbo-diesel). The synthetics just do not offer any real (as opposed to advertising) advantages
for the vast majority of every-day applications. (I might spend the considerable extra money for Mobil 1 0W-40 European Car Formula IF I were operating my Sprinter consistently in sub-zero temperatures or if my Sprinter was equipped with ASSYST and I wanted to push the change intervals to the limit rather than change every 8,000 - 10,000 miles).
TIP: DON'T PUT ANY OIL IN YOUR SPRINTER THAT DOES NOT HAVE THE APPROPRIATE MB SPEC NUMBER PRINTED ON THE CONTAINER !! There are a lot of so-called premium brands (including most formulas of Mobil 1) that do not meet the MB spec for your vehicle. The MB engineers have specific reasons for their requirements; trust them over slick advertizing.