Automobile(s): 2013 CTSV Coupe , 2012 Escalade, 2014 SRX Join Date Apr 2003 Location H.V., Pa/Town of Webb, NY Posts 13,018 Re: Is it safe to use 87 octane Sean, what you are doing is making 90 octane by mixing, it will not hurt anything except performance and fuel mileage, the lower octane will mean that the computer will retard your timing enough to not allow your truck to knock.