Well today I tried out a couple mix tests of Vallejo Airbrush Thinner with a couple of Acrylic brands that DO NOT work with Mr Leveling Thinner.
first up I tried Model Master Acrylic. Good results. No clumping, gumming up, or any other such mishaps.
I then tried the red headed step child of my acrylic paints, Humbrol Acrylics. Again Good Results. No clumping, gumming up, or other mishaps. There was a bit of grittiness that separated from the paint, but I also had that result with Model Master Universal Acrylic Thinner, and that combo airbrushed reasonably well.
I’ll try actually airbrushing the paints in the next few days and post those results here once I have finished that experiment…
I use retarder and flow improver depending on the temperature. I haven’t shot much, as I only recently started using it and when I did I shot it straight up without any additions. Too cold to spray here in the winter, as the garage is my spray room with the door open.
I have been using propriety thinners for my paints with no adverse reaction, Mr. color with Mr. thinner, Tamiya acrylic paint with their acrylic thinner and lacquer thinner. Recently purchased Vallejo AK 3rd generation acrylic paint and acrylic thinner (AK11500) very pleased with my results. no paint tape lift etc. your tests are interesting which paint did not do well with Mr. thinner? thank you.
Humbrol. Mr Leveling Thinner turned Humbrol into a gel. I haven’t tried MLT with Model Master or Polly Scale Acrylics, as I’ve read that similar results happen with those.
MLT with Mr. Color is my go to Gunze marketing is strange, Mr. color this and that…goofy, at least their not on the proper pronoun lunacy too bad Model Master ist kaput, they still make Testors, now owned by Rustoleum. they dumped floquil etc.
I TYPICALLY thin with distilled water with my various acrylics…especially the cheap stuff when I want a different texture. I have the Vallejo thinner…just don’t really need to use it much.
Tap water being an issue all depends upon where you live. Here in AZ we have extremely hard water, so a water softener is a must if you want your plumbing fixtures to last. But even then it tends to give funky results when thinning acrylics. I use distilled water for some of my thinning with acrylics such as Humbrol.
Always use distilled water when thinning paints, especially for the airbrush. Even the best tap water still contains chlorine, fluoride, minerals, lime, iron, and other deposits that are not harmful to humans. They can, however, be deposited inside your airbrush and slowly decay it. These deposits also effect the pigments and binders in the paint. So, spend the dollar and buy a gallon of distilled water for thinning.
FWIW - sometimes depending on specifics, hard water doesn’t seem to play nice with applying decals in my experience. At previous residence learned that the hardway. Switched to bottled water for decals and didn’t have repeated issues of water spots etc. Pretty sure that tap water would have been less than ideal for thinning.
Gunze marketing is strange, Mr. color this and that…goofy, at least their not on the proper pronoun lunacy 
too bad Model Master ist kaput, they still make Testors, now owned by Rustoleum. they dumped floquil etc.