I got the xylene at Lowes. I was looking for toluene which is a more difficult chemical to get (I have not found it locally, but have not looked up the automotive paint stores). Everything I read says if it is compatible with toluene then xylene will work fine, but the evaporation rate is slower with xylene. I also found posts on RCG that stated toluene, and MEK would work, with MEK being a "faster" thinner than toluene, and that MEK encouraged blushing. (I also got MEK at Lowes, but didn't try using any.)
I am using modern clear, and vintage blue (Pactra). They both seem to thin and cure just fine with the xylene on board.
The curdled, corn starch thickened clear I added the lacquer thinner to went to smooth and usable after adding xylene too. That mess seemed to cure properly too. I used it up and sanded it off already. It was pretty dilute with maybe 1 part dope to 4 parts thinner, if that.
From everything I have recently read the primary reason for a cocktail of thinners is to control evaporation rates. There are other reasons such as certain thinners better separate solids, prevent or encourage cross linking, effect gloss, "blushing", and surface hardness, and more... but I didn't understand the chemistry in that article beyond if the finish is not as desired try a different thinner, or blend the compatible thinners.
Now I am almost out of colored dope and not done with the job. I have two planes to paint before monocote, and only one is done. The "formula U" I have is polyurethane and not dope. I found even less information on that stuff, effectively none on thinning, if necessary.
I may break down and order some dope. Rustoleum is a choice still as these are planes for learning not show.
Phil