I just received the letter in the mail yesterday for my 99 ML320. Considering what it costs to get the A/B service performed, I can't imagine complaining about the miniscule cost that is added for using synthetic oil.
Reminds me of some time ago reading about a new Rolls Royce model that came out where the factory decided to use mineral oil instead of brake fluid because according to the spokesperson, it was "cheaper"! (Ya think that's a real concern with the typical RR owner?)
Anyway, the whole idea about using synthetic oil is that it requires less frequent oil changes, which align well with the service intervals for the later Mercedes-Benz models.
But there are enough posts here that deal with the regular vs. synthetic oil question...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car