I always thought that using octane booster in your fuel was like using snake oil--big claims with little results. But I read an article in European Car magazine where they tested some and it showed that there were real gains. They tested NOS brand octane booster, but I bought some concentrated Pennzoil (comes with 4 little bottles) from Canadian Tire because they didn't carry NOS. Afterwards, when I was at Lordco I saw that they did carry NOS-brand so I bought it for later use. Since 94 octane is very hard to find in Washington State, I was worried about using lesser fuel in the STI during the trip down to Alki Beach, Seattle. Instead of carrying a gas can in the trunk (like some of the other guys), I bought a few bottles of octane booster to try out. I know with the Cobb Accessport , I could have simply flashed the ECU with a real-time map optimized for 91 octane, but I wanted to see how smoothly the engine would run with the additive.... I'm glad to say, that it worked well ...