About every third fill-up (at half tank intervals), I blend 5 gallons of Sunoco 110 with the 91 octane water that Chevron sells. I stick with Chevron because their blend is at least more consistent than other options I have tried. $6.58/gal for the 110, at a pump. The car does run a little tighter on the blend, and a side benefit is that the scent from the pipes makes my garage smell GREAT.
-Dean