The fluid is a petroleum product, and the plastic of the bottle is also a petroleum product. I'm sure you've seen the other discussions on the forum about "plastic compatible" oils. I've had Lionel 2909 smoke fluid (old formula) partially evaporate and get gummy at the bottom, although some others here said that wasn't possible. I've also had other brands cause the dropper to swell (and then the dropper had to be discarded.)
I would say a tightly-capped glass bottle is your safest bet. Squeeze out your dropper after every use. The train-shaped white bottles made out of "hard" plastic, as used by MTH and LGB, seem less prone to disintegrate (although I have no way to test whether the lighter fractions of the fluid have evaporated off.) My $.02.
