Originally Posted by cartuner

You mention black smoke and sweet exhaust fumes. To me that's running rich and burning coolant. If you are burning oil it will most likely be blue smoke. Yes, if you're burning oil you'll see it on the spark plugs. I would check to see if you have a external leak. 1/2 a quart per 100 miles seems to me as a small leak externally. I would check the cam seal area again, I have seen many leaks from that area even after replacing with brand new seals. The problem is not the seal, its the area around the seal that leaks, applying some silicone around the seal prior to installation will likely cure the leaking from there.
One more thing did you tell them the cause of the problem was a downshift? Giving them all important information will make it easier for a tech to diagnose your car. I'm speaking from experience.