The new oils are really good, but I think most recommended for MB are 0W40 so still that 40 weight. MB's spec is really quite strict, it's a better oil than Rotella...

Plenty of new diesel MB running in Sprinters with crazy mileages and I never have heard anything at all out of them, and that is with them asking for oil change at 15 000 or 20 000 miles or whatever.

Don't forget how much faster a 0W oil gets to the engine compared with a 15, I put 0W30 "German" Castrol in my car, and compared with the unknown 10W30 or so that was in there, the cold startup was amazingly faster. At -20 F ambient the car started as if it was 60 F outside. You're saving all that startup wear.