Heed the warning WMichaels put in his post. My former managers wife used nothing but 87 Octane in her Mercedes SUV (an ML 350, I believe) and it killed the cat converter. It took a while to do it, but it did. Well, that's what the mechanics told him anyway and it cost several thousand to fix. It seems possible, if there is a lot of predetonation, then the engine will be burning fuel less efficiently, leading to higher amounts of impurities in the exhaust.