Would someone explain why, according to the Ford maintenance schedule, some items are supposed to be replaced after a certain mileage or a specific period of time? For instance, Ford specified air cleaner element replacement after 30,000 miles or 3 years. The mileage requirement is understandable, but what magic event happens to require replacement of the air cleaner at three years? It seems to me that the air cleaner elements should last indefinitely until they are dirty enough to replace. Inquiring minds want to know ....