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I have been using Costco 91 grade in my past cars (bmw, jaguar) for close to 8 yrs, and for the past couple of yrs I have been using it in the Corvette and Mercedes. Never any issues. I purchased the Corvette new 2yrs ago and at first ran it for 3 full tanks of 94 grade, then went back to Costco for their 91. I have to say that I have not noticed any difference, maybe if I tracked or did the 1/4mile I may have. For my driving style 91 seems just fine.....
