I don't generally waste time with penetrating oils on stubborn threads. If it's seriously attached, nothing much is going to change that other than heat. If it's a bolt, I give the head a good clout with a hammer first to shake it up a bit before applying torque. If it's a nut, heat to red and try to undo. If no success, repeat heat but smack a flat with a cold chisel to expand the nut (preferably supporting the other side in the process). Alternatively, drill a small hole through the nut, parallel with the bolt / stud. Easy then to split the nut. Or, if sufficient room, place a hammer against one flat and smartly (no gentle tapping here!) hit the opposite flat with another. Then reverse and repeat. That expands the nut and it should come undone easily. Final option is to threaten it with Blue Dot (a reloading powder). Not that this would achieve anything, it's just a matter of instilling fear in the nut.