Try a search on this website for the word Xylene & you may also wanna try Homebrew Race Gas.
Here's the ratio I use:
1 gallon xylene : 3 gallons 93 octane unleaded = 99 octane final mix
I always mix up a 12 gallon batch & add ~2 oz of Marvel Mystery Oil to replace the gasoline's lubricants that get diluted by the xylene.
Doug C.