Carbon buildup on the end of the spark plug can prevent easy removal even if not on the threads. Of course, exposed threads allow a better foothold for carbon and make removal more of a challenge. Heavy carbon buildup is not normal and can indicate an overly rich mixture, excessive oil in the combustion chamber and/or too many short, stop and go trips.

As for whether one must visually confirm penetrating oil is saturating the threads. Not necessarily, but if you have a boroscope........ The threads are near the bottom of a cast well in each cylinder head. However, penetrating liquid introduced here may not end up at the spark plug threads. I say, just go for it. It's only oil. Will assume sufficient restraint to avoid flooding the area.

Unthreading the spark plug away from the sealing surface allows an accurate squirt of lube to access and saturate the threads and hopefully dissolve whatever is holding onto the plug.

The term "straw" refers to the thin hollow plastic tube attached to the spray head and used to direct liquid contained in an aerosol can.

Sherwood

Sugarwood -
Your comments are correct, with the borescope I could see the oil was getting in/around the threads of the plug. The straw was used to deliver the oil. After a few attempts of playing around with everything, I was able to see the oil coming out of the straw and soaking the plug thread area.

larrym -
What I should have done is taken a picture of the plug I removed and posted it in the forum (my bad). I can describe for you that the last 3 to 4 threads on the plug had hard carbon deposits between the threads. They were cracked/crushed from the removal but it still took a little bit of wire brushing to clean up the threads.

Sugarwood? How sweet. :)

after all that - you reused the plug!

Wait, why did you need the boroscope if you could just eyeball the oil level in the clear straw?

correction:

Apologies to the real Sugarwood. I incorrectly assumed DKM was affectionally addressing me.

Sher wood

LOL, Sherwood's comment now makes sense!