Do you mean seal of the tube to the cylinder head? I was just reading about that, in one of my cylinder head refurb pdfs, repair manual excerpt. It specs the sealant and instruction for tapping it in. Can post in a bit.
@hardish , did your sparkplugs look like mine shown in below pics? I am not sure if i have this tube seal problem which leads to oil on the plug. I am not even sure if the residue on my spark plug's thread is oil. I thought the oil came from having oil coming from PCV valve, PCV hose, into Air Intake Manifold, and then into engine.
Most likely, you have a leaking spark plug tube seals. There are 4 seals inside the valve cover which block oil from entering the tube. They are a pain in the butt to change. I have a set of 4 leftover from my engine overhaul gasket kit if you want them. Mine were not leaking (and they are easily accessible later on) so I elected to not change them.