I had the same problem on my 408 and it turned out that the engine was burning oil because of excessive crank case pressure. The excessive pressure was causing oil leaks and under throttle the pressure was pushing oil past the piston rings back onto the cylinders. My engine was force inducted and that might have been a reason for some of the excessive crank case pressure. The two open air breathers on each valve cover weren't enough to get rid of the pressure. But, after I came up with a PCV related system of relieving the crank case pressure, the smoke from the exhaust and the oil leaks completely stopped. For what ever it's worth, check for excessive crank case pressure before tearing down the engine.