I had this happen once when some dummy (me) used RTV on the manifold gasket. The gasket slipped at the bottom of the center two ports on each side and sucked oil. The plugs were real oily. When I pulled the intake, it was easy to see that the gasked was oil stained and leaking.

If your heads have been milled or the block decked, the intake sits higher relative to the head ports so that might make the bottom part of the gasket leak. There isnt much compressed gasket area down there.

Did you use any sealer on the intake gaskets? Try them dry...easy to change.