I have used older true oil thinned with mineral spirits in a plastic cup without an issue. But this was for smaller projects and not a gun stock I had spent lots and lots of hours on. I agree with the wisdom of using a new bottle for that purpose. I also store mine upside down.
The sucking in of the sides over time is fairly common. Sometimes the biggest challenge can be getting the crusted over lid off. I’ve often had to snug the plastic bottle up in a vice, while pressing down on the lid, and turning it with a pair of slip joint or channel lock pliers to remove the lid. It has a safety lid on it that crusts up pretty easily. A little solvent or turpentine should return TO’s consistency if slightly thick. But as stated, I do this in small batches poured into a clear plastic cup from the bottle, and mix with a few drops of solvent at a time, which is easier to get what you want - rather than trying to pour something directly into the brown TO bottle.
TO is easy to use and dries fairly quick. As with almost all finishes, several thin coats are better than one or two thicker ones. But it can be overly glossy for my tastes, no matter how it’s applied. And I normally find that I have to cut it back with steel wool or a scotch bright pad.
