Originally Posted By: BHopkins
Miles.
To change by looks, you have to open up the airbox, and on many cars also remove the air filter. Every time you do that, you take the risk of nasty stuff getting past the filter.
If the filter is a little dirtier than normal, the risk is if it gets severe enough i'll have a loss of power. No risk to engine damage, as a dirty filter traps even smaller stuff. If I take it out at the designated mileage and it isn't quite as dirty, it's OK. I've already opened the box, so put a new one in. Stay on schedule so I know when to do it again.
It is my belief that my engine is much better off if I leave the filter alone as much as possible. A filter minder would be the best option, but I've never used one. I see the value in a filter minder on a big rig where filters can be very expensive. But when I can buy an air filter during NAPA farm days for about $8, it's easy to change them on a regular schedule.
This.^^^^. Actually both . I know about how many miles I can go till the filter's outside pleats get dirty so I don't check and replace the filter till that mileage. 20,000 for me. Every time the filter is checked there is a possibility dirt can get into the intake.