If an injector were over fueling, wouldn't you be able to pull the spark plug in that cylinder and tell if its running rich too? Also, what if he has a bad fuel pressure regulator or fuel pump? If one of them is causing low fuel pressure, but not low enough that its stops running, wouldn't this cause issues with the spray pattern in all of them? If your spray pattern isn't correct...or I guess I should say if the fuel isn't atomizing properly, it will not burn completely, possibly causing the fuel dilution in the oil. I know, its a stretch, just trying to run all the scenarios through my head. I work on diesels for a living, and I've seen injector tips get dirty or damaged that the truck still seems to run ok, but you get a little black smoke out the stack because while that injector is still working, the tip can't properly atomize the fuel meaning that cylinder can't burn all the fuel its getting, even though its technically still getting the proper amount. This causes smoke like a cylinder that is being "over fueled."