Tung Oil

Not in my experience - though you can still use it as a gel in hand-rubbed applications if it hasn't lost all it's liquid-ity.

I have never tried to reconstitute tung oil. However I have used mildly gelled product on small projects and it worked fine. I wouldn't try this on your prized, polished walnut table top.

This came up a while back. The oil will tend to gel as it is exposed to the air-gap at the top of the container, as you use it. I used to keep a box of glass marbles in my shop. I dropped a few marbles in the can as I used the oil to maintain a small air gap. Seemed to help. When I emptied the can I dropped the marbles into a jar of spirits, wiped them off and reused them.

Can't hardly find marbles any more. Kids have to have a $400 electronic game to feel loved these days.