Unless your air filter is ripped there is no reason to replace it. Even if replacing it, do not forget to oil it with something like this:
http://www.amazon.com/Uni-Foam-Filte.../dp/B0022ZL5DW
If it's dirty, just wash it really well with dish detergent and oil it back up. If you don't have specific oil and are in a pinch (like needing to get to the auto parts store to buy foam air filter oil), spray some WD40 on the air filter and massage it thoroughly all the way through. I like to do this in a zip lock bag, keeps my hands from getting super oily.
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