I've always had issues sealing up factory heads with a stiff valve cover. The factory head castings are typically not consistent around the sealing surface of the valve cover. With a stamped valve cover that flexes, the valve cover will bend and seal, even in the low spots where the head castings aren't perfectly flat. Stiff machined valve covers don't flex, so no matter how tight the bolts are there's going to be a variance of pressure applied to the VC gasket and oil usually pushes its way out in those spots. When I went to a modern aftermarket aluminum head, the sealing surfaces are spot on, and they sealed up much better with a variety of valve cover types.