while it is nice to think green, one reason i bought an aftermarket filter, i have heard bad things about k&n filters. 1. the oil used to coat the filter has a tendency to foul the maf if the filter is over-oiled, which i understand is not hard to do. 2. the k&n's have been said to give poor filtering performance, the daylight test has been mentioned several times on several forums - hold the filter up to the light and you can visably see holes that dust particles will be able to pass through. actually, many people say the filters work better after they have seen some use after a cleaning, the dirt fills the holes and helps the filtration :Wow1: not so good. some guys have said they looked into the intake tube and saw evidence of dust, as well as dirtier oil, further lending to the idea they may not work as well as claimed. i have however heard good things about true flow filters,
http://www.trueflow.com/ a few guys on here use them, and i think i will be switching to one shortly. speaking from personal experience, i can surely see the need for a spare filter. i was following a friend on a dusty road, and my crappy cold air kit (soon to be replaced with the factory air box i bought) was inundated with dust which tripped the check engine light. when i took off the cone filter to clean it and the maf sensor, the intake tube was noticeably dusty. i then thought, crap, time to find a new approach. bring on the snorkel!!
i might as well add to the list - spare inner and outer tie rod ends and cv's. you can do the same as rick suggested with your belts and hoses, if you replace your tie rod ends as you should, which is before they fail
you can use the old ones as spares, and i just use my old cv's with torn boots which i plan to fix as spares.