A tip for the day: How to handle the removal of stubborn paint caps, especially for oil paint tubes. Short answer is wax the threads of the tube.
This is an old tube of Holbien Aqua-Duo “Sepia” oil paint. It’s made with asphaltum pigment (NBr8,) which is a nice dark shade, but I believe they’ve changed it to another pigment now. The bad news is when it dries it’s most certainly like asphalt – rock hard. You may have experienced problems like I did when trying to unscrew the cap. It just wouldn’t budge by hand. Doing so caused the tube to twist, and possibly tear, which would be a bad thing, or another problem is the weak plastic caps easily break. I had to take a couple pairs of pliers to grip metal tip of the tube and the plastic cap, and then slowly and firmly twist it off.
The next step was to clean off as much dried paint from the threads and the inside of the cap as possible with solvent. I had to use some of acetone on the threads. Next I wiped a bit of beeswax on the threads, and screwed the cap back on. The wax will make it easier to twist off in the future. If a tiny amount of wax happens to get into the paint, it won’t cause any problems. It would be wise to do this now on tube caps that haven’t yet gotten to be a problem.