Yes, octane matters. Running too low of an octane can cause you to detonate. Since you drive a GSR, you need to run 91 octane because of the compression ratio. 100 octane is fine to run, as long as it's not leaded gas. Leaded gas will poison your catalytic converter.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
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