Originally Posted by paroxysym
I could never understand this thinking. Octane has absolutely nothing to do with the additives a petroleum company gives to its fuels. Fuels are of different octanes to work with the compression of a particular engine. This is directly related to the fuel's volatility. On the compression stroke, you increase pressure on the fuel, and, therefore, increase its temperature. In a high compression engine, this could lead to pre-ignition of the fuel (catches fire before complete piston compression and firing of spark plug). That results in "engine knock." This is why there are higher octane fuels, because they are less volatile and resist pre-ignition (detonation).
Save your money on the 91 octane, and just buy a couple of cans of fuel system cleaner for a couple of tanks. I've always heard great things about Techron, but try whatever you think is best. I've tried various cleaners and can't say I feel any difference between them, but maybe I've just never had a fuel system issue for the additives to make any difference.
Good luck.
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