Jerry,
Thanks for taking the time to describe how a spark plug may become unserviceable. Good description! The carbonized oil sounds like what was happening on those old worn out oil burner engines of my youth. In my teenage years I bought a gimmicky motorized spark plug cleaner tool from JC Whitney that "sand blasted" the plugs. It never really fixed the badly oil fowled plugs.
From your description it sounds like the important factors to look for on a spark plug is the amount of deposits forming on the ceramic portion and erosion of the center electrode. It is presumed that using correct heat range spark plugs matching the use pattern should minimize carbon formation. Working with engines that are not totally worn out will help too!
Paul
