Refinery LPG valves need to be lubed. So refiners inject x ounces of heavy lubricant per thousands of gallons of LPG.
A new LPG bottle may weigh 18 pounds. Ten years later it may weigh 24 pounds with zero pressure. Guess what's inside sitting on the bottom? Refineries have liquid traps that almost work. The gas dries out the lubricant over a long period of time and it becomes wax like.
I have cleaned a 100 pound cylinder with a gallon of acetone soaking for a couple of weeks. Then a jouncy ride on a desert track, dig a hole and tip the cylinder upside down with the valve open.
Gloves and disposable clothing. The odorant was so intense it smelled like an industrial accident -- not recommended.