One persons stuck might be another's loose. Try a 3/8 breaker bar after soaking for a while in the oil, alternate tightening then loosening.. back and forth with moderate pressure.
Don't try the following unless you are a professional and know your tools.... I brought mine out with a 3/8 impact turned way down. You can turn an impact down till it is barely slapping and it has a better chance to bring the plugs/bolts out safely in my opinion than constant pressure with a breaker bar. I brought every bolt out safely on my heads even the exhaust manifold bolts alternating forward then backward with a 3/8 impact turned down. Didn't break one bolt. I'm replacing them because they are rusted to all end but my point is the mild hammering of an impact mechanically stands less of a chance of pulling threads on dissimilar materials (rusted steel plugs, oxidized aluminum heads) than constant pressure like from a breaker bar.