wd40
If it came to that the penetrating oil would be more appropriate
Wd40 is a good multipurpose oil, but penetrating oil is thinner which enables is 'penetrating' ability to get deep inbetween the threads, and also evaporates which makes it a little less messy. Wd40 is a better lubricant (not paintball lubricant of course) but for unsticking a barrel is ideal