If the heads were not perfectly leveled when they were milled (or milled too much), in conjunction with using the cork end seals, it is plausible for the intake to suck oil. There are also a few other things that could do this; leaking (dislodged positive) valve seals, guides too loose, valve stems out of spec. The least expensive would be to reinstall the intake manifold, and use sealer instead of the cork end seals. The sealer will seal better than the cork, as the cork could stop compressing if the gap is too small, and not allow the intake to completely seal. Myself, I use Permatex Ultra Grey.