I will be running Xylene this season as an additive.
My father has a painting business, and he has a discounted price on gallons of the stuff.
As you may know they are used in the painting industry.
Anyway, there is a good thread about the subject on dsmtalk.com. I can't find it now, but maybe a search over there will come up with something.
If you mix toluene or xylene with regular unleaded gasoline, then you will have problems with it eating rubber parts inside the fuel system, and other problems.
This problem is eliminated by using an additive such as "Marvel's Mystery Oil". This adds the lubrication properties to the toluene/xylene and makes it friendly with the fuel system.