Following up on my earlier post, I got the oil tank out, and found pretty good evidence to support my idea that the sealing surface of the tank itself might not be perfectly flat. You can see a small dimple which would be at 6:00 as installed, right about where the sealing ring would sit. It isn't deep, but the o-ring might not press down into it too far. Seems like a good explanation.
Even though I've got another tank coming, I'm halfway tempted to rough that surface up and fill it with some JB weld or something that could be sanded flat, if only to test my theory. Probably not worth the hassle, but it would be interesting... We'll see how the incoming tank looks first.
Mike
PS - the exhaust gas analyzer is a good idea. I'm thinking my next project will be a carb re-jetting, and possibly looking at larger venturis, to help this thing breathe a little better. The Zeniths are still jetted for a standard 2.2T engine (whereas I'm at 2.4, high compression, and running E cams.) Then a serious exhaust analysis could help dial things in.