1 spark plug is carbon fouled

all of a sudden ,one morning my Vehicle started running rough.I figured id replace the plugs and found 1 to be carbon fouled.I replaced all eight and within minutes the same 1 plug is all black and sooty.I tried a hotter plug and replaced the ignition wires and hate to spend any more $$$ with trial and error.Any help would be greatly appriciated

What cylinder is it in?

Is it the one of the rear cylinder's?
I have seen the plug in my #8 cylider with oil on it.
One of the valve gaskets might be leaking oil into the cylinder.