bala: the k&n (and i guess the bmc) filters are reusable. instead of taking the old filter out and putting in a new one, you clean it and oil it (spray it with a liquid that cleans the filter, then another liquid that helps capture particles). put it back in and it's like a fresh filter.
some people have had issues with them where (as other's have said) if they are over oiled, the oil gets pulled towards the engine. the main problem with that is the MAF sits just behind the filter so the oil collects on the MAF causing it to need a replacement. a not inconsequential sum for a rather cheap trade-off. (reusable filter or buy a new filter every time)
personally i have stuck with the one use filters.
How to clean a K&N air filter* warning: will make you want to play tetris!
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