Intake manifold pulling oil
as long as we're throwing around WAG's and SITD's... maybe an issue with the head locating dowels? maybe the heads didn't go back on the sides they were taken off of and one of them has just enough shift/out of spec on the pin to throw the whole thing? that could explain the issue of having the problem after the removal of the heads.
i didn't see if you answered but is it problematic on both sides or just one bank?