Diesel engines need air to work. This is why turbochargers are used to effectively increase the power output of an engine. A turbocharger is used on many diesel trucks to pre-compress the air before it goes into the cylinder where it is compressed even more. After a cylinder is close to its maximum compression, fuel is injected that instantly ignites due to the high temperature and pressure of the air. (In diesel engines, fuel and air are not premixed outside of the the combustion chamber.)
Diesel engines, therefore, are extremely dependent on the intake path of the air and the composition of the air. The more air that can be compressed inside a cylinder prior to combustion, the higher the efficiency of the engine.
This dependency on air has its drawbacks. Air is mostly made up of nitrogen, which serves no benefit to the combustion process. Although very little of it changes chemically during combustion, some of it changes into nitrogen oxides, NxOy, which cause environmental pollution. As temperature and efficiency increase inside the combustion chamber, so does the concentration of NxOy compounds in the exhaust.
Many newer turbo diesel engines being used in trucks are now using novel methods to reduce the levels of NxOy byproducts. When you see a truck that needs either BlueTec liquid, urea, or it must use ultra-low-sulfur diesel (ULSD), then that engine is probably using a new design. The USA instituted ULSD in the fall of 2006 and mandated that some model year 2007 trucks be designed to reduce NxOy.
One of the many design problems with lowering NxOy levels is that exhaust gas recirculation is commonly be used to lower combustion gas temperatures. While cheap and effective, this can cause soot buildup in the oil, which is why the American Petroleum Institute CJ-4 (API Service) specification is designated for USA 2007 MY and later trucks. The newer motor oil is designed to be able to disperse higher soot levels, making it more tolerant of EGR engine designs.