For starters, the phenomena of concern are preignition and detonation. No such animal as "predetonation".

There will be no loss of power going to a lower octane unless detonation occurs. **Unless** the engine in question has a knock sensor and retards ignition/lowers boost. Not a problem in an '85 380SE; no turbo, no knock sensor. If anyone read my first post, I advocated sneaking up on the lower octane fuel. I'd say that qualifies as "thinking twice".

mpolli, I would also think twice before taking any more advice from that dealer who told you to reduce octane to reduce knocking. He hasn't a clue. I've heard people tell me that "Premium is a hotter fuel so it makes more power". Wrong on both ends.

Lastly, there is no "insurance" factor in running higher octane gasoline than your engine needs. It won't give more power or better economy, either. Nor, like one commercial implied, will the pretty girls smile at you because you spend too much on gas.

Regular is fine,

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Norm in NJ
Next oil change at 230,000miles