Mike,
There are much better penetrants on the market. To get the oil worked down
into the threads I have had some luck by gently tightening and un-tightening
the stuck item back and forth repeatedly. Some heat might also be good.
Here are some google results:
http://www.faqfarm.com/Q/How_do_you_remove_a_stuck_spark_plug
http://www.findarticles.com/p/articles/mi_qa3828/is_200507/ai_n14824048
I just hope it is not cross threaded.

Ken Lewis
http://neksiwel.20m.com/