Despite what K&N may claim, they can't get around the laws of physics. There are only two ways to get more air through a filtration system: 1) Increase the surface area of the filter or 2) increase the size of the openings in the filter material. Since the surface area of K&N filters designed to replace stock filters is the same, that only leaves option 2. Using a K&N or similar type filter won't cause your vehicle to die a sudden death in a short amount of time, but if you keep your vehicles long enough, they will let more dirt in than the stock filter will, which will lead to more wear over time.
Depending on the vehicle, things can get even worse. The stock filter on a 2006 Duramax for example, is a round cartridge resembling a large diameter of blue paper towels. The K&N replacement is their standard cone shaped pleated filter. In tests, because of the stock filter's design which increased surface area via its design, the stock filter out performed the K&N considerably. Yet the same tests proved the K&N let more particles through. One of the important factors in this test is the fact that diesels draw far more air than gasoline engines, which directly equates to more dirt. The test results simply reinforced what had already been explained to me about the physical properties of filters.
Use K&N if you wish - it's your choice - just don't fall for some of their advertising bologna. Remember, snake oil by any other name is still snake oil.
