W124's run on 91, but I have run 89 without any problems... I use Mobil 1 10W-30 in both of mine, one has 193K and the other 79K. I live in Kansas, and do not drive them in freezing weather. Which oil to use is really a subjective thing, people get very worked up over it...

The leak could be a head gasket getting ready to go. Get it inspected by someone who knows W124's before purchase. I would be concerned about that.

Good Luck,

-GH

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1992 Mercedes Benz 500SL
1998 BMW 528i
2003 Honda Element
2002 CBR600F4i
1977 Coachmen Motor Home