Too bad that all light duty vehicles do not come equipped with filter restriction gauges from the factory to extend engine life and save owner's money. If you check any heavy duty filter manufacturer recommendations, you will find that filter restriction cannot be determined by sight. Here is a quote from a presentation in a 2001 Filtration Conference by Donaldson Company employees titled "Nanofibers in Filtration Applications in Transportation":
"Consumers must also make maintenance 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 air filters rely on the formation of a dust cake to improve the performance of a filter media, over-servicing can lead to dire consequence from inadequate engine protection.
Air filter over-servicing is common for 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 lube businesses to recommend more frequent air filter changes. Given the frequency of oil changes and air filter replacements at quick lubes, it is expected many light vehicle air filters are changed much more frequently than manufacturer's recommend.
While this over-servicing phenomenon is frustrating to those who understand filter media performance, the filter industry has not sufficiently educated customers how air filters function. Not surprisingly, filter manufacturers, distributors, dealers, service stations, and quick lubes have economic incentives to change and sell more air filters. It is also understandable that consumers believe air 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 engine air filters are over-serviced, a typical automotive filter may operate for most of it's 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."