I dont think there would be anyway to get that much oil from a combustion chamber blowing past a plug. how would it get into a cyl and not smoke out the tail pipe?

why is there no ash build up on the plugs from burnt oil?

if it did have a crack in the threds it should still only push compression by the plug.

if it had a crack from an oil passage to the threds. if it had a crack from the crankcase area of the heads to the outside of head along VC rail.

when oil puddles around the intake bolts/pockets, the oil is wicking up the threds. this is because the intake bolts go into the open area of the oil crankcase. r&r of the bolts adding rtv and re-torque to specs it stops the leaks.

you could try some brush on antisieze on the plug threds to see if that stops oil from puddling up around the plug.

IMHO, I just think the oil is from the VC rail above the plug pocket.

or it could be from the intake end rails and windage blows it around and back down the side of head to plug area.

looking at the plugs there seems to NOT be that much oil in the cyls to blow up the threds.

I have found loose plugs that where blowing bubbles in the mud truck when it had been in the deep water,i just torqued them down some more to stop it.

see what the baby powder shows, be sure to clean it down good with brake clean to remove all oil traces.

do as before. run engine a while then let it sit a while with the powder around the plugs and side of heads above plugs.

do the water test for bubbles when you wash the powder off.