Engines can use abnormal quantities of oil if they were not broken in properly when they were new; if unsuitable oil viscosities or harmful additives were used during their lifetime; if ester based synthetic oil (all synthetics except Mobil 1) was used during the first 5,000 miles; if the engine was switched from ester based synthetic to petroleum oil at any point in its life; or if different brands of oil were mixed in the engine between changes.
If your truck has the 4.8 or 5.3 liter motor; these engines are known for a design flaw which causes sludge to build up in the oil, and creates excessive oil consumption. Many, many people have had this problem with those 2 engine models; and many of those engines have been repaired or replaced under warranty. But I have never heard of these repairs or engine replacements making any difference. The problem always comes back. This is because the cylinder heads on these motors were redesigned so that the engine would warm up faster and thus produce lower emissions; but this head design also creates hot spots in the coolant passages which cook the oil and create high temperature sludge in the oil when it passes over those spots.
In order to stop this problem; the engine would have to be rebuilt or replaced, and fitted with cylinder heads that have a different design. Trick Flow makes the lowest priced heads which match this application, at about $1,000 each. http://www.summitracing.com/parts/TFS-3