Doc, I have a friend with a Ford F-250HD with the 460 with the exact same symptoms you describe. It too, burns clean, no fouled plugs, green coolant, etc. He's at a loss besides suspecting the valve guides. Personally, I think it's the thin wall castings Ford uses for their engine block - no rigidity = a bit of blowby around the rings, especially after the cylinder walls have worn a bit... Please post what you find, as I'm sure many here are interested!