My wife managed to put diesel in my Jaguar XJ6 once. Luckily only in one tank. (It had two tanks). Took a bit of effort to get the diesel out. She actually ran it through the motor on the highway too, then quickly switched the tanks back. I was FURIOUS! I mean, come on, that took real talent. The nozzel doesn't even fit in the tank fill neck.

I believe your car has a knock sensor, which should retard the timing and prevent detonation (knocks) before they happen. This is probably the reason why you don't think its running very well now, the motor retarded the ignition timing. I wouldn't think that going from 91 to 89 RON would make that big of a difference around town though. You know midgrade is simply a 50/50 mix of regular and premium.

My W124 300E does not have a knock sensor. My wife topped the tank off with over 3/4 of regular before a road trip. I drove to the autoparts store, added octane booster, then after I was down 1/2 tank filled up with premium. Never had any worries, and lesson re-learned, never let the wife fill up the car.

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On the plains of hesitation bleach the bones of countless millions who at the dawn of victory, sat down to wait, and waiting -- died