I would never use a K&N on anything I own. They basically flow a bit better at a penalty of letting a lot more dirt to pass. You will most likely not feel the difference, nor will you detect a mileage increase, but you will allow a lot more dirt into your engine.

Here's a quote from the article:

"Compared to the AC, the K&N “plugged up” nearly 3 times faster, passed 18 times more dirt and captured 37% less dirt. "

Yowza!

ISO 5011 Duramax Air Filter Test Report