It's difficult to answer your question without knowing the service history of your car -- specifically as it relates to the transmission. For instance, if you kept up with the transmission service intervals recommended by MB (you didn't state how many miles were on your 500E) and had the fluid flushed, as opposed to just having a drain and fill procedure done, switching to a synthetic transmission oil probably won't hurt. This also assumes that you don't have 300K miles on your car too.

We work on a lot of MB transmissions here, but as a general rule if we are servicing a high-mileage vehicle or one where the trans has not been regularly flushed, we do not recommend switching over to a synthetic ATF. If you do decide to make the move over to a synthetic, make sure that the shop flushes the system and adds a conditioner to the system too.

I hope this helps.

__________________
Bill Shaw
www.CurrysAuto.com