Welcome, Baldy. I'm gonna look forward to hearing what fixes this one!

Like Tech & Dave said, I'm curious about the clearances between the intake you're using to the heads and front/rear valley machined surfaces. I think I'd set the intake in place without gaskets and see how it sits paying attention to any angled gap at the ports (like Tech mentioned) or gap at the "China walls" of the valley. I'm thinking that you may find that it's sitting on the valley walls, and standing proud from the heads a bit, maybe at a slight angle. Before you lift it off, put in four of the bolts to the head surfaces to see if that changes your gap at the valley ends. Your highest vacuum is at idle, so it's gotta be sucking oil that's running down into the valley area, seems to me.