Since you can turn them about 5-10°, I give hope to using the penetrant. Work the plug back and forth, turning the plug out a little more each time. The back and forth motion will work the penetrant into where it's needed and may save you.

Typically we don't use penetrant, but as Ray mentioned the threads are coated with an antiseize compound to help the next time around.

Another trick I heard is to warm up the engine (I haven't tried it). Aluminum expands more than steel under applied heat.