They are wire mesh and cotton, impregnated with oil. It is far superior to paper filters. It can be cleaned with a special solvent, let air dry for a half hour, re-oiled with a spray bottle, and be like new again. The KNN air filter is a particularly good change for the Dodge 5.9 diesel engine. Exhaust systems, stock, are often very inefficient, and cause power loss because the exhaust tubing is convoluted, crimped, and therefore inhibits the free escape of exhaust gases.
Mufflers can also be an obstacle.
After market options, such as Banks mufflers, can offer larger diameter tubing. This apparently may be most true for Chevrolet. The newer Dodge and Ford stock exhaust systems may pose fewer problems. A diesel engine simply needs to be turbocharged. At higher elevations, a non-turbocharged diesel becomes highly inefficient.