I have a really good friend that is a Real, honest to goodness Powerstroke mechanic. He said in one of his classes they were told that Open element air filters such as the K&N did not filter enough particulates out. He also had it verified shortly after that by an emissions inspector here that told him that any vehicle that has an open element air filter is more likely to fail because of significant wear in the bearings and ring facings. I asked my friend what the spec that ford has is, and he said it was two mil. He also said that the K&N let up to 10 mil particles throught he filter. I called Airaid and talked to an engineer, and he said that their spec is also 2 mil. For what it is worth, I did not tell him what the Ford spec was ahead of time, and did not even tell him what I had.
Scott