Look those plugs over when you pull them... If only one's fouled you've probably got a stuck ring. (I'd shut it down right after it stops smoking and pull the plugs)
If the PCV valve is pulling
oil look in the vacuum line right where it connects to the manifold. If there's more than a slight film the valves bad.
Here's two more possibilities: Cracked block or blown head gasket, these will also cause low compression on the effected cylinder(s).