I also have an '88 300E. About a year ago I tried running mid-grade and stuck with it. It seems to work just fine. My idle may be a little better since then. It very well could have to do with additive differences causing a cleaner fuel system.
Have a great day,
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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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