There are a variety of reasons why a car or truck may be burning oil, most of them related to wear and tear on valves and seals around the engine. Most of them can be prevented with regular maintenance.
If valve seals on the cylinder head go bad, oil will be allowed to flow down into the combustion chamber and will be burned.
The valves are guided by a chamber called the valve guide. As they are worn down gradually through use, oil may be able to flow through the gaps and into the combustion chamber.
If the Positive Crankcase Ventilation system becomes clogged by carbon buildup, then this will pressurize the oil pan and force oil into the fuel delivery system.
When the piston rings wear out, excess pressure from combustion flows into the oil pan and can push oil into the fuel delivery system.
Many of these problems can be prevented by basic maintenance, the most important of which is to regularly change oil and air filters.
