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There are 4 ways for oil to get into your cylinders.
1) Blown turbo seals - ruled out by plug indications, all plugs would be equally fouled if it was this.
2) Bad/frozen rings - have you run a leak-down test? It will isolate any cylinder leaks including rings, valve seats and...
3) Blown head gasket - see #2, have you tried the tissue paper trick on the oil filler? With the engine running, place some semi porous paper over the hole (hold onto it). There should be a slight vacuum indicated by the paper being drawn into the hole. If it bulges under pressure it indicates an abundance of blowby gasses which could be a clogged PCV system and/or some of the leaks associated with #2.
4) Leaky valve seals - Eliminated all of the above? This is what is left.
Your plugs indicate a single cylinder problem, I seem to remember reading someplace that cyl #4 plugs will indicate leaner/hotter combustion under certain conditions, you may want to run a search for that. I suspect you may be right about the valve seals on #1. Don't read too much into the fouled state of #2 & #3 condidering the auto Rx and redex treatments. If you are reusing your plugs, why not swap #1 & #4 and see if the deposits burn off in #4 and appear on #1? Just a thought.
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Bob Kraushaar '94 945T, '88 240, '84 242Ti, '88 300 TE, '89 560 SL, '68 Shelby GT-500 KR
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