Seeking banal opinions on engine air filter replacement

i'd still run them. Where I live 60k+ is fine. Now on dirt road you might need to change it every 10k

Now if you are chasing every last fractional bit of a HP a fresh one now might make sense.
Those look fine. Maybe vacuum and reuse. The dirt looks fairly dry, so I think a good amount can be removed using a vacuum.
Where are you and when is pollen season? I would try to limp through the "dusty" season on an old filter so the new one will have a chance at life.
Install a restriction gauge; change filter at 11" restriction.
On my previous Toyota that took 42K miles.
Install a restriction gauge; change filter at 11" restriction.
On my previous Toyota that took 42K miles.
Yup, this. This is the way to do it, as @Imp4 also mentioned earlier.
Is there any real citeable source that says a dirty filter is more efficient? Or is that one of those poorly thought out ideas that people think works in theory but fails to acknoledge that a dirty filter creates less effective surface area for the air to move through which with the same flow rate means more pressure is being applied on the remaining clean spots on the filter thus causing more particles to be forced through the filter that may not have been if the filter was new.

Here you go.

The best air filter document that I know of is “Nanofibers in Filtration Applications in Transportation” by Employees of Donaldson Company given in 2001. Here is a quote about consumers and light duty vehicles: “Consumers must also make maint decisions about the air filters in their cars. This user group is relatively unsophisticated in their knowledge of filter maintenance and function. Air filters are perhaps the most easily serviced but least understood parts on a vehicle. Other common replacement parts on a vehicle such as tires, wiper blades, oil and oil filters work best when new. However, this is NOT the case with conventional air filters. Since most (dry) air filters rely on the formation of a dust cake to improve the performance of a filter media, over-servicing can lead to dire consequences from inadequate engine protection. Over-servicing is common in light vehicles. Light vehicles are generally not equipped with filter restriction indicators. Air filters are often inspected by maintenance personnel during oil changes. In spite of the typical manufacturer's recommendation of at least a 30,000-mile change interval, it is common for dealers, service stations and quick lubes to recommend more frequent air filter changes. Given the frequency of oil changes and air filter changes at quick lubes, it is expected that many light vehicle filters are changed much more frequently than manufacturers recommend. While this over-servicing phenomenon is frustrating to those who understand filter media performance, the filter industry has not sufficiently educated customers on how air filters function. Not surprisingly, filter manufacturers, distributors, dealers, service stations and quick lubes have economic incentives to change and sell more filters. It is also understandable that consumers believe that filters work best when they are new (like other parts) and have a clean appearance. In spite of the best intentions of automotive mechanics and consumers, typical automotive air filters are notably inefficient at capturing particles less than 5 microns in size. Several studies have shown that particles between 1-5 microns cause engine wear, which will lead to increased engine emissions and shorter engine life. Because many car engine air filters are over-serviced, a typical automotive filter may operate most of its life without the protective benefits of a well developed protective dust cake. As the filter becomes dirty by visual appearance (and the dust cake finally starts working) the filter is often exchanged.

Original source:

Donaldson is a somebody in air filter world:
Both filters are about 2 years old with about 20,000-25,000 miles on them. It's understood that replacing them is cheap insurance. But I also know that some dirt improves filtration. You can see that there's some sediment in between the pleats, but not that much (hopefully it's visible in the last 2 pictures). What you think, are these at the peak of the filtering capability or beyond and should be replaced?

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The dirt improves filtration analogy is because it’s partially restricted… that means good fresh air also.