Here's an interesting FAQ from the Federal Trade Commission:

http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm
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Thanks for the link Ardub, that was what I was talking about.

If we had 91 octaane available here that's what I'd use all the time but since we don't I mix my own. Sometimes I even fill about 2 thirds of a tank with 93, stop the pump, put my credit card back in and finish filling with 89. It's a pain so I usually fill it with 93 and when it gets to half full fill again with 89.