The quality and condition of your engine air filter will have no effect on MPG. It may have a perceptible effect on top end power. 40K miles is time to change. My vote is for whatever is OE quality or better and cheapest.

 

When I still had the OEM intake I chose to install a K&N filter because I wanted to clean my own filter every 6 months or so. I also liked the little growl I could hear after the install. It was cheap fun.

 

Why? If you can see light through it, then its not clogged and there is no reason to change it. Is the gasket damaged or any part of it torn or broken?

Air filters are very much an "on condition" replacement item because the amount of dust in the air varies so much from place to place, so a set mileage interval is not appropriate, except maybe for periodic inspections.

If you drive on dirt roads all the time, it may only last a few hundred miles, and should be inspected often. Thats probably not in the US/Canada/Japan/EU owners manuals, but might be on other continents. My old Mexican VW's manual recommends checking the air filter daily if driving on dusty roads.

 

I change my cabin air filter yearly, here's why

In October 2012 I started smelling something from the HVAC system... rodent excrement. So I Googled the info for how to replace the filter and that's how I found FitFreak. It turned out that a mouse/rat had been using my Fit as a hotel, the cabin air filter was full of turds. The little rat-bastard even chewed some of the sound insulation behind the glovebox but fortunately he didn't chew anything else. I replaced the filter, changed where I park, and set the air selector to "recirculation" overnight. Problem solved. I still wonder how that critter got inside.

The last time I changed the cabin air filter, Nov 2013, there was no evidence of critters but it was full of dust, dirt, leaves, and seeds. If I were to have put that filter in a tray of water near a window it might have sprouted trees.

Last but not least, one more anecdote. When I asked a local Honda dealer about replacing the air filter they quoted $80. Haha, no. $50 to install a $30 part in 5 minutes? That's $600 per hour! Moral of the story, do whatever maintenance you're able to do yourself.