In my own experience one of the places for oil burning is bad valve oil seals. If they were installed incorrectly or damaged at installation time it will cause oil to pass the valve seals into the combustion chamber. You can actually see it early in the morning when starting the truck. This may or may not show up in the condition of the spark plug being affected. Oil can also be burn at the piston rings which or course is more expensive to fix. As suggested in another posting visual inspection of the plugs could tell you which piston is having the problem. Wet and dry compression tests will indicate also if the burning is happening either at the valve guides (valve seals, possibly) or the piston rings. You may also want to replace the PCV valve it may help.