I'll side with sims assesment of the vlve guide seals.I learned the lesson 55 years ago of the value of proper valve guide sealing of the engines which liberally oil the valve stem area of which the "oil through the pushrod engines" are a major player.The need is compounded when this heavy oiling is increased by higher than normal oil pressure which many performance engine builders go for as a bit of added insurance for their bearings.
Many times the plugs may look normal short term but run the engine 5000 miles in daily driving conditions and the telltale ash/carbon will be there bigtime.
What I don't understand is why engines without the good seals will many times have very good oil control.Probably has to do with natural sealing ability of the deposits on the stems--and that leads to a thought of the enthusiasts myth that prompts the fear,"I shouldn't do a valve job on my old engine because it may start using oil".I assure you if the cylinders/rings are doing a proper job of oil control as they are, the correction of valve sealing is not going to hurt--but disturbing the valve stem sealing can.(just my 2 cents worth-other opinions are encouraged but may be ignored).
The factory didn't machine the guides for Perfect Circle type seals untill mid 60's and I'm not sure when they started installing the seals from the factory.
Some of the worst engines for this problem for me was the 170/200 CI sixes from the blue oval.I've cured several in mustangs and falcons--they would usually have acceptable oil economy and no smoke but would build a "clinker" in those plugs and short them out.Probably was related to the oil additives used in those days(not to mention the blue oval creator).
