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Generally speaking foam filters will flow more air compared to pleated cotton filters (depending on who makes them). Look into reviews such as UNI, ITG, Pipercross, etc. Almost, all dirt bikes, sport bikes, etc. come with foam filters for extreme conditions. ARMA is known to be a good brand as well (they might use one of the above MFG to make their filters - it's pretty common).
If you are looking for a filter with a bit more surface area and better quality than K&N look into BMC. I ran one on my 340i and it worked very well. They have better pleating, better fit and gasket material, and slightly more surface area. I had thought 340i filters fit X3 M40i too...if so you can buy the BMC filter right from ECS Tuning.
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