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I too looked for this info about a month ago (and couldn't find a specific filter for the E90), but after reading reviews on the K&N filters, I think I'll stay away from them for now. From what I could ascertain, the way it increases the airflow into the system is by allowing more air to pass through the filter and thus much more potentially damaging dirt/pollutants into the engine. There was a study done that compared many different brands of air filters, didn't give me any confidence at all!!
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