I've completely taken apart my 08 MBP a couple of times, including the logic board, fans, and the fan exhaust(whatever it is called). putting any kind of filter will drastically block the flow of air. You'll just have to carefully blow it out once in a while.
If you don't have a temperature monitoring program already, you should get one. And give it a cleaning if the idle temp exceeds 60 celcius or so.