Nglitz is 100% correct. Octane is a measure of knock resistance and that is it. It is my understanding that 10 or 15 years ago some gas companies were making claims about mileage and power and were fined by the government for false advertising. You will notice they do not make these claims any more. They tend to make a lot of "toothpaste" claims such as "no gas cleans better" or "helps to remove deposits that can rob power". But any gas with additives (required by law) can help to remove deposits. Yes some have more additives than others. So we have fancy names for these such as Techroline, Pro Power, and V power etc. And I have been told that using a higher than necessary octan can INCREASE emmissions. Here is a funny story: My friend is very picky about her 2003 531i. She always used Shell premium. Then one day they started advertising that it contained "V-Power" Her manual didn't say anything about V-Power so she switched to Chevron!!

Another thing, both me (last week) and one of co workers (last year) were advised by our local dealer to use regular (87) in out Mercedes (me C230, him E320) in both case to REDUCE knocking. Go figure. (Any comments on this are welcome since it makes little sense to me)

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra