AV Vallejo has been very secretive about its thinner composition. The paint is a polyurethane mix and does not dissolve with alcohol. Water does not wash it well either.
I recently got my hands on a bottle of Butyl Cellosolve and realized that it smelt the same as the AV thinner. I got a 500ml bottle for GBP3 to make my own thinner for AV.
Here is the result:
A is with AV's own thinner and T is the test I ran using my own mix of 10% Butyl Cellosolve, 30% IPA and 60% water.
The levelling effect is the same, the drying times are similar and the paint texture looks similar.
When I used it to wash a paint brush after painting with Vallejo paint, the paint washes off and does not gum up.
Butyl Cellosolve is a butyl ether and used as a solvent for acrylic resin. Its other names are 2-Butoxyethanol, Butyl glycol or EGBE (ethylene glycol monobutyl ether).
A 500ml bottle costing GBP3 can make me a 5 liter batch of thinner or airbrush cleaner.