Dont know about its source but I saw a refernce to this which is a so called "Independent" test of Air Filters -- Its on a Duramax Forum -- Gets to a point with some of the aftermarket stuff you just dont know what to believe. As I said, my Dad had one on his Ford and had good luck with it.
Here is the Link...
http://www.duramax-diesel.com/spicer/index.htm
73
Mark, W3ZI