I'm sure the topic of changing the air filter has been beaten to death - is there a topic that hasn't been? But I can't find a thread this specific - so my question is . . . My buddy needs a complete motor rebuild for his XR400 ($1500) - his mechanic says it's because of a never-changed and poorly installed air filter. I guess they were scooping dust out of it by the handful...anyway, this guy says to change the air filter after every ride - have two and keep one cleaned and oiled and put it in after the ride and clean the other one etc. You get the picture. I clean and change my filter but not nearly that often. So do I run the same risk with my 2002 WR426 if I just change it when it looks dirty?