Plugs fouling, left side only

[RET_COP]

Motorhead, like I said in a earlier post, the oil problem is a problem. You might be able to fix the rich condition especially on the left bank but oil contamination is going to mess up your observations and sensor readings.
You said oil on both sides.
It took me a year to figure out my oiling problem, I took it in steps, [I was still driving the car a lot], and eliminated one thing at a time. Rings would have been my worst situation and I was saving that for last. I checked:
Plugs, two sets
intake runners from carb to valve to see if it was some or all {big cams and open plenums can fool you}
PCV even eliminated it and went with two breathers
Intake Gaskets twice
Intake to head angle
Valve Guides
Intake Rocker arm studs.
It turned out to be rocker arm studs. On ported SBF heads they are tapped into the intake runners. I tried to seal them with ARP thread lube, and it just turned to grey goo, I tried blue loctite, and that was ok. I changed to Ultra grey after a through cleaning with break cleaner and a small amount of Blue Loctite and problem solved. I never thought oil could wick through those studs.
I had my intake off so much I could make the change in a few hours.