I'm not saying that the engine isn't worn out or even that the PCV valve is the only problem. What I AM saying is, that PCV valve is plugged up.
My ex-father-in-law loaned me his brand new 66 Impala convertible (cherry red w/ white interior) to make a trip, and during the trip the PCV valve plugged up. Blew oil all over the engine compartment, smoked like a son of a gun, rattled like crazy, lost four quarts of oil in no time. I barely got it to a shop, where they said the engine was gone - until a mechanic looked at it and diagnosed the problem.
You replace that PCV valve, and I'll bet you the oil loss goes away. Now, as to whether that's all that's wrong, it's hard to say. You didn't say anything about how long the truck has had the problem, how it ran before the problem started, whether or not it's always smoked, so it's not possible to say everything that might be wrong with it.
