I've heard of people using Toluene in gassers before, supposedly improves octane, energy per gallon, etc. Avgas actually uses a significant amount of toluene - Chevron avgas is around 14% toluene.
Mixed it at about 5-1 with gasoline, and it did seem to increase power significantly in my quad - I was able to beat people up sand hills at the dunes that I never could before. I was actually able to pull a full gear taller up some of the hills. Be aware, this quad is far from stock - with overbore, high compression, cam, bigger valves, bored throttle body, etc. so results may not be consistent on more mild tunes.
Toluene has a R+M/2 octane of 114, and mixed at 20% with 91 octane gas should yield around 96 octane. Energy per gallon is higher, due to increased fuel density - around .87g/ml, as opposed to gas with .72-.74. At 20% mixed with pump gas, energy density should increase from an average of .73g/ml to arount .76g/ml, so a 4% theoretical power increase is possible from energy density alone. Factor in anti-knock ignition retardation with the octane increase, especially in high perf engines, and the potential becomes much higher.
Never heard of it being used with diesels, though. Anyone have any experience or input?