Allan,
If you haven't already done so, do a search on this and you'll find literally hundreds of posts on this very subject.
MB does recommend (or maybe mandate) use of synthetic oil on new vehicles with FSS (flexible service system) since they have extended drain intervals.
FWIW I do use Mobil 1 synthetic in my 400E just because these engines tend to run hot and they are pretty expensive to service -- a valve job alone runs about $6000 per Stu Ritter -- so I want to give myself every possible edge ...
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Steve
'93 400E
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