1991 300SE Owne, as you can see everyone has an opinion on the oil and oil viscosity that is best for your engine. If you do a "Search" using "Oil" or "Oil Changes" you will find many threads discussing the subject ... hours worth of reading.

The factory fill for that vintage engine was 15W40 mineral oil. Check the index in the back of your Owner's Manual under "Fuels, Coolants, Lubricants, etc." for the page with a chart that- shows oil grades (weights) versus ambient temperature.

Synthetic oils have swell agents that prevent gaskets/seals from leaking. Since you have been running synthetic oil in your 1991 300SE without any problems it's fine to continue using synthetic oil in my opinion. I use Mobil 1 15W50 in our engines of that vintage and have never had any leakage issues, even when switching to synthetic oil after 100,000 miles. The Service Booklet recommends oil changes every 7,500 miles and I follow that frequency religiously. (I also use Mobil 1 15W50 in my John Deere tractor engine. During a recent camshaft replacement at 1,065 hours, a known Kawasaki engine issue, the service manager said the original cross hatching on the cylinder walls was visible. Meaning the engine showed no/little wear.)

Bottom line: I would continue with synthetic oil. George Murphy, the Technical Director of the Mercedes Benz Club of America, endorsed Mobil 1 during a tech session I attended in 1998 so I have been using it since then. Our local MB dealers use Mobil 1 for all oil changes.

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Fred Hoelzle