Your handbook should give details, although the specs will probably have been superseded. In my 1996 C200 l used to use Mobil Super S which l think has a slightly different name now. It's a semi synthetic 10/40 grade and l believe the dealers used to use it sometimes. I still use it today in my 2001 SLK230 which also uses the M111 engine and I've never had any problems. You could of course opt to use a fully synthetic and more expensive oil but l don't think there's any need for that this engine. Just change it regularly and you'll be fine.
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