My experience with thinned enamels is that they go bad surprisingly quickly, with a shelf life measured in weeks rather than months. Enamels are kind of funny, because I've had some that congeal in the bootom of the jar, even without having been thinned and with the cap screwed on tightly. Recently though, I have found that lacquer thinner seems to be helpful at rescuing paint that has globbed into the bottom of the jar. I suspect that because lacquer thinner is hooter than regular enamel thinner, it is better at disolving and separating the pigments.
The interesting thing is that acrylics seem to be a lot more immune to these effects. I have a little plastic lidded cup that I have some highly thinned MM rust + Tamiya flat base, and no matter how long it sits and how much it separates, I can always mix it up again and shoot through the airbrush with no clogging.
Andy