This is getting off topic, but Snap-On's newish 2 year air tool warranty is pretty much the best in the industry. Brass fact is Air Motors, Hammers and Anvils, and any moving part actually will wear out with use, so how could they warranty it? Buy one tool on special and expect a lifetime supply of tools that will wear out in a few years?
The only air tools I have seen that are life time warranty are cheapo mastercraft ones... but they're much worse brand new than a worn out Snap-On or IR...
For that plug... yeah, just give it the beans. Penetrating oil probably won't do much good. Heat will, but there are seals that might not like it. Short whapps with an impact should break it free, work your way up in power if you're worried about damaging anything, but like said, replacement parts are available worst case.