What are the chances that the exhaust system is so full of unburnt/partially-burnt oil, that every time you whack the throttle open, you heat the exhaust enough to burn the existing oil inside the pipes/muffler?
I've just re-read this entire thread; and I have no deep insights.
The two things that trouble me are 1) the likelihood of oil in the exhaust; if you'd just drive the car (hard!) for a week or two, you might find that your "smoke" gradually disappears as the built-up oil in the exhaust system clears-out; and 2) the presence of oil on the #6 intake valve, but not in the #6 intake runner. And yet the #6 plug was not one of the really "bad" looking plugs which were #7 and #2.
In MY driveway, I'd be verifying:
1. Cylinder leakdown with a leakdown tester.
2. Actual sealing of the valve-cover bolt holes. You said you dropped cut-down bolts into them...but I would put a vacuum-tester with a rubber cone on those holes and see if they hold vacuum. If they do, great. If not, they need to be sealed better.
3. One wonders if you could do the same with the rocker-stud holes, from the port side. You'd have to pull the manifold again, and I don't know if there's room to cram a rubber cone for a vacuum-tester into the port in a way that the rubber seals to the hole in the roof of the port.
I'm starting to wonder if the problem is rings, and oil on the backside of the valves is the result of intake airstream reversion during overlap, or as a result of poor valve sealing. A leakdown test would absolutely verify valve sealing.
Sorry, that's all I got.
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