[Theunz]

My 05 F250 has an air filter restriction gauge, as I think the newer diesels do as well. It has never moved and my truck has 125,000 miles on it with only one filter change. My feeling is that if you're not showing a restriction, than your filter is flowing all the air that your engine requires.
I used to own a 90 F350 with an aftermarket turbo and air box on it. That gauge would show a restriction as soon as you accelerated even moderately with a brand new paper filter. I switched to a Amsoil foam filter and it took a hard acceleration to move the gauge. I then switched to a K&N filter and the gauge still moved, but not as much. I finally resorted to adding some small ram air tubes and holes in the top of the air box to allow all the air needed. That tells me that the gauges are very sensitive and that the K&N does indeed flow moe air, filtering efficiency notwithstanding! YMMV