I'm sure the woodworkers here already know this, but leftover linseed or tung oil needs to be stored with no airspace in its container. If there's air above the surface of the oil, it will oxidize and harden on top. I use clean pebbles or glass marbles to displace the air and bring the level to the top of the bottle. Another method of minimizing the air in the container if it happens to be plastic is to squeeze the bottle before putting the cap on. Soak any oil-soaked rags in water and store them in a sealed paint can or other container to prevent spontaneous combustion from oxidation of the oil.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!