Paint thinner is typically a toluene/xylene mix Toluene is used in both Avgas and regular gasoline as an octane booster so it will certainly combust well enough.
However, adding too much to fuel would seriously reduce lubricity and will !!!! up many engine parts including valves and fuel pumps pretty quickly. Also, the solvent effects of the xylene may cause problems with the fuel piping.
Finally, only approved additives are allowed to be added to the fuel, and paint thinner is not approved (though toluene and xylene are present in many additives that are approved, albeit in fairly low proportions)
So, if illegal fuel, fecked up engines and fuel systems are not a problem to you, feel free to use it
