Don't ever introduce thinner into the original paint bottle it will ruin the paint faster. I have never had a problem with long term storage of Tamiya acrylic paint. Model master enamels will go bad much faster. Gloss even faster.
Anything thinned for an airbrush in an airbrush bottle can't be stored that long. Never pour that back in the original paint bottle, it will end up ruining the original paint.
General tip is to wipe the rim of the bottle clean. Right down to the glass. Then use a Q-tip to wipe around the inside rim of the lid wiping up any paint there. Then hand close very tight. This insures a tight clean seal.