So i'm filling up my tank yesterday, and I see another guy with an M3 also filling up at the pump next to me. I couldnt help but notice he was putting 87 octane in his car (I always use 91). So I asked him why he uses 87. He said that he "used to use 91 and since he switched he cant tell any difference in performance, and it is top tier gas". Im wondering what if any affect the use of lower octane would have on the car long term. I dont intend to switch, no way in hell.