All oils will soak into metal. Just about everything you touch has some kind of petroleum base in it. whether it be natural or manmade. When they produce iron from iron ore then burn the impurities off, and melt it down, and add whatever they need. Then they take those impurities and make **** with it.
I know this because my family owns a foundry. We bought it, and decommisioned it, and cleaned for the EPA, but during the decommisioning I learned how they casted cylinder heads. I actually learned how to cast my own intake. I have never a complete intake manifold, but I did a couple pieces for my truck, as well as some turbo pedestals for my truck. It' pretty cool ****.