A compression test is definatetly in order and if you have the ability to do a leak down that would be good too.

Check the hose for the PCV valve and the hose from the intake tube to the valve cover. If either one of those is clogged, a functioning PCV valve will do nothing to recirculate crankcase vapors.

Is the truck blowing blue smoke? If the oil rings were that bad, you would be loosing oil into the combustion chamber too.

Good Luck,
Michael