Spark plugs
 

What does it indicate, if when you pull a plug, the threads are oily, and some oily sludge exists slightly outside of the compression ring, in a situation where the plug boot fully seals around the plug hole? (The plug hole was clean otherwise, so this didn't seep in from outside).

Who did the last plug removal before yours and why did they do it?
As sloppy mechanic doing a wet compression test?

That would be me. I'm fairly certain I got the plug in tight at that time. The compression ring was crushed anyway, and the plug wasn't removed after that until it's recent replacement.

It was just a shot. Couldn't think of an obvious mechanical explanation, so I looked for human intervention. Could it have been a spec of crud under the compression ring preventing a 100% seal?

Hard to tell. The sludge only went slightly past the ring, there wasn't very much on the metal body at all and none as high up as the nut, but the ring itself is completely sludged over.

If the bottom (mating) surface of the ring is gummed up with sludge, it must not have had a good seal. If the ring is crushed, there must have been something trapped under the ring when it was seated. No?

Quite probably. I just can't find it in all the sludge. :D

Are you sure it's oil sludge and not some decomposed anti-seize compound?

These were my plugs. The problem? My thermostat was cooling in excess, making the mixture too rich.

http://i185.photobucket.com/albums/x...lasantigas.jpg

http://i185.photobucket.com/albums/x...lasantigas.jpg

There wasn't any anti-seize on the factory plug, and I didn't use any when I changed the plug. That second plug I pulled out had the sludging problem.