My very close friend was a top 10 Pro ATV National Motocross racer just a few years ago. The engine builder (One of the top engine builders in the ATV world who even Factory Mikita Suzuki Hired to build there engines) who built his engines did some filter R&D. He tested different foam filters and different K&N style filters. The major thing they learned is that the K&N filter and the foam filter were very close in overall flow. The K&N style actually per se flowed better, but they always have a hard plastic or metal top. Air will not go through the top. Foam filters can flow through the top thus creating more overall area for air to be sucked into the engine. The K&N was used in less dusty conditions while the foam was used in dry conditions. All filters were oiled and had a sock on them in addition to the prewrap on the foam. If you run on pavement, it would be best to oil the filter a very small bit. Use filter oil though. Dirt, you must oil the filter and oil it well. I did oil my filters twice in 2012 but am less concerned as I run on asphalt and engines are rebuilt somewhat frequently...
