I personally spoke to a Mobil Tech and was told." if you car is a high milage car or diesel they do NOT advise you to switch to syn.
Here's something you may find interesting. I took my '86 560 SEL into a Jiffy Lube ( I know, I know, stupid move) for an oil change. They informed me that they will only put syn oil in MBs in order to keep the factory warrant valid. I informed them that the warranty on my car was long gone....but they said "Corporate told us to only put syn. oil in ALL MBs that we service." And they wanted to charge me $57 to do it. I said no way, bought my own filter and took it to another quick change place. Bad move...they overfilled the engine and when I got home I noticed oil coming out from around one of the valve covers. Fortunately they only overfilled by a small amount and the oil burned off quickly. But I learned a good lesson....no more "quickie lubes" for my car. It is now going to get all the oil changes at my regular independant MB shop. May cost a bit more but I know they know how to to the job properly the first time.