Maybe I'm getting paranoid in my old age, but when I was notified that the FSS cars now required synthetic oil the first thing I thought was MB found a problem with the new V6 & V8 engines (such as soft cam lobes) that may not lead to premature failure if synthetic oil is used. I know the FSS is used on other engines also but the vast majority must be on the new V engines, and calling for a switch to synthetic oil on all 1998 and later cars means they wouldn't have to disclose the problem on just one engine type.
I read an extensive technical paper(SAE#981443)touting the value of the FSS(known as ASSYST outside of USA), as evaluated by MB in over 2.5 million miles in Police, Taxi, and "Nornal" use, and extensive dynamometer testing with non synthetic oil. For them to now call for only synthetic oil is not consistant with the findings outlined in the SAE paper, and leads me to suspect a posible manufacturing problem.
Just something to think about.