Testor's enamel

I only use thinner if I airbrush. If it's semi liquid, goopy or whatever after you paddle mix it, then it's old paint... toss it out. 
I keep an eye on my flat, semi, satin and gloss black, rubber black, steel, aluminum, silver, signal amber and red. Other than that, I buy as I go, not unless I have it and it's in good shape. I won't ruin a model or an airbrush for $3.00

Just get an aluminum paint pallette and add a little thinner to the pallette at the time you're going to use the paint.  Its almost impossible to predict the long-term chemical reaction anything you put into the bottle will cause.

Thin only enough to use.   If you thin the entire bottle it will be useable today/tomorrow but when you put it up for a month you will come back to a couple useless mess

Like he said, adding that stuff in the bottle will lead to rapid end of the bottle. You’ll get more mileage from that bottle by mixing up what you need for a session at that time. You can use generic paint thinner, or Testors own brand for good results. 

I use them in precisely the same fashion, since they're handy, cheap and reliable. I thin with regular "paint thinner" or mineral spirits...avoid turpentine, since the gums in it don't seem to play well over time.

Lacquer thinner can sometimes be used to thin for individual applications, but "in the bottle" it tends to start the break-down process.

It's really tough to quote any hard and fast rule with these paints, since they seem to change formulation almost constantly. In general, they tend to degrade quite a bit faster than they used to...usually turning goopy or even solidifying.

Cheers