I would be careful about using too much penetrating oil, but it might be your only hope. I once read a warning that if it leaks past the plug, it can do internal damage to the engine. I think I read that here somewhere.
If it was my vehicle, I'd try to gently loosen the plug with a 16" breaker bar so I'd have the leverage to do it gently. I might make a gentle attempt to loosen it, walk away from it for a while, and then take another gentle effort to loosen it again later, repeating the process several times. Think of the plug like the lid on a jar of pickles. After someone has tried to get it open, he hands it to you and it opens with very little effort, and you say, "You must have loosened it."
Another idea might be to try gently loosening the plug on a warm engine instead of a cold engine. Maybe the metal surrounding the plug might expand slightly more than the plug itself.