The refrigerator bulb in my brain came on today. In one of those videos, there was tons of oil coming out of the #2 and #4 cylinder pushrods, and not much out of the #6 or 8 pushrods. I know I mentioned letting it run longer, but if you did and the same thing kept up, something may be amiss with the lifters.
How does the metering work in those lifters? Does the lifter control the flow of oil to the pushrod with a fixed orifice or a disc? Is there a chance one of those lifters is bypassing too much oil, and not pumping up correctly? Meaning, all the oil is going right out the pushrod, and not staying trapped in the lifter body to take the slack out of the plunger?
I know its a long shot, but if you don't find anything else, then you would have to look at the lifters. This is after tie bar contact, lifter bore condition and valvetrain geometry is verified.
Here is some lifter 101 for reference:
http://www.victor-aviation.com/Hydra...on_Matched.php
https://www.youtube.com/watch?v=b2RZqIETIpQ