I haven't used Testor's enamels for years, but now that Jim mentions it, I do recall that there was a drying oil in their metallics at one time.
This can also lead to problems with shelf life if there is a small loss of solvent. That's when the retarding effect (not true retardation) of the oil can cause havoc. I still also suspect a high pigment load.
Metallics generally have a lower shelf life than other paints, regardless of system, because of the oddities of the pigment, and the problems with formulation that makes necessary.
Rattle cans of metallic have a very definite shelf life (buy only as much as you will use on one project!) but it is usually very easy to tell when they have gone "off."
Off topic: Congratulations on your success in college, Jim! Glad to hear it!