It is possible for oil to leak past the exhaust valve guides and be blown out the exhaust, as said by 4ontheFloor. Either worn guides or leaking seals are suspects, and in that situation, you don't see any evidence on the spark plugs. That whole problem is what led to the development, in the late 1950s, of the umbrella seals that fit over the valve guides. I agree with desertdog, a quart in 200-300 miles should leave a lot of oil spatter on the back end of your truck.
Harvester
