I will ditto that over maintenence is detrimental. Every time you open the system is a chance for dirt to get on the inside but worse yet a dirty filter actually cleans much better than a clean one. As dirt is trapped in the element the pore space actually becomes smaller, filtering out finer particles that could get through a clean filter. No filter is 100% effective but a dirty one is more effective than a clean one. The trick is if the filter becomes too dirty it starts to restrict the air enough it could reduce engine performance, but that is what the restriction indicator is for.

If you really wanted to be a purist and protect the engine regardless of cost the best plan would be to run a brand new filter till the restriction indicator came on, then throw the filter away and replace with a new one. As I recall this is what IHC promoted for a while in the late 60's or early 70's when I was in school. Their theory was that more engines were damaged by improper filter cleaning and excessive cleaning than anything else.

We kind of take a middle of the road cleaning schedule. If we are in for other maintence such as oil change, etc (we do oil at 350 hrs) we will clean the filter if we are not pressed for time, just so we don't have to take it out in a dusty field when we are in a hurry doing field work. Combines often will go till the indicator goes off but if we are in black dusty (mold) corn we will do it at 50 hr service just so we are not stopped in the middle of the day - we have a pretty good feel for how long the filter will go in certain conditions. Otherwise we make sure and blow them out during our winter service and change them every other year. Combines every year if we have had to clean them several times. I think Deere recomends every year but with the hours we put on the tractors I feel every other is ok for us.

John