Smoke Fluid Longevity?

My experience with a couple of different brands, is that the lighter molecules evaporate away and it ends up thick and gooey.  That would also explain why you're observing decreased smoke production.  The stuff that vaporizes most readily (and makes visible "smoke") is precisely what evaporates most over time.  On the plus side, a drop or two of the thickened stuff probably helps to protect the wick and prevent it from running "dry."

I just keep mine in a box in the train room.  I have no idea if storing it in the fridge would help it last longer.  My thought is to buy the smallest bottle and use it quickly!  When you see smoke fluid on the shelf, you can shake or swirl the bottle a little to gauge the viscosity, which might be the best indication of how long it's been sitting there.

I'm not a chemist, and all of the above is just my opinion.  I would love to hear from someone more qualified that actually KNOWS.

I know I've used stuff that's 4-5 years old and it still smokes fine.  That's a sample of one.

We have four bottles(different flavors) we bought when the layout was constructed thirteen years ago.  As far as I can tell, the fluid is still good.  No scientific way to test today's performance vs. performance years ago but the MTH units put out lots of smoke.  John in Lansing, ILL

Evaporation? It must. Wicking dries out over time during non use so where does it go. I would say evaporation. I’m not a chemist so I’m only guessing. 

My take: Every smoke fluid I have tried (practically all of them) has lasted indefinitely in the container. I have 15 year old Train America Studios smoke fluid that is every bit as good as it ever was.

Oh - but now that I think about it, there's just one exception. Lionel Premium Smoke Fluid, which does evaporate fairly rapidly once the container is opened and no matter where you store it, including in a completely dark space. 

Why that would be so I have no idea - all I can tell you is that it's an observable fact. 

I have smoke fluid that is very, very old and it still works fine.   It does evaporate out of the engines when they sit on the track.  I solved that problem by capping the engine stacks, steam or diesel, with a small piece of aluminum foil after I'm done running them.  It's amazing how many more times I can run them before refilling them.  As a side, I had a bottle of Lionel Pine, the only flavor I ever had this happen to, that ate the rubber stopper and migrated out the top of the bottle.  I re-bottled it into a used MTH loco-shaped bottle and after time it actually migrated through the sides of the container.  I'd like to have someone explain to me how that could happen.

I have 50 year old Matx smoke.  Still has the smell only Pepe LePew would love.

Keep everything in a cool and dark place. I notice that it helps keep the scent on the smoke fluid.

I have left some smoke fluid resting on its side. It ate the eye dropper rubber up. Stand up your spoke fluid containers.

You should also always empty the dropper before putting it back in.  It will eat the rubber bulb if it sits in it.

About 20 years ago, I bought a case of Lehigh Valley (LVTS or something like that) Christmas scented smoke fluid from a guy who lives/lived in Emmaus, PA. He was retiring as I recall, and I loved the scent so much I went to his house to buy it. It was and still is great stuff, and I've only made my way about half-way through the case. It looks like its brand new.

I want to say that MTH used his smoke fluid - LVTS, until he retired or they started making their own.