I just got gas. Only after replacing the nozzle in the pump did I realize I just filled my tank with 87 Octane instead of the good stuff.
AND I have a 200+ mile trip up to the mountains tomorrow.
Does the M5 have knock sensors (which would automatically retard timing/reduce throttle/adjust mixture) to prevent knock? Will I be OK? Should I add in a bottle of that after-market octane booster stuff, or might it do more harm than good?
Closing comments - I pushed the car hard with the bad gas - no ill effects except a little down on power. Evidently the engine management computer just does the right thing, which is probably to retard the timing a little bit.
So - in case this ever happens to you - don't worry about it!