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I used to use a mmt based octane booster in my old 2002 z28 only at the track. Over a year of use(in summer, approx 8 days a month at track), my plugs looked like the inside of a oyster shell, that rainbow translucent look. Car ran ok, but I did notice some issues over time(may not have been octane booster additive) but 02 sensors failed at 35k original miles on car too. Honestly, comparing times with and without the added octane, I did get my best times with the additive. Once I seen the plugs I stopped using the octane booster and paid for the race fuel at track.