There are many SAE papers that verify dry type air filters become more efficient with use as long as the seals are air tight. As examples see #952557, Total Filtration by Marty Barris of Donaldson Co. and #2001-01-0370, Discriminating Tests for Automotive Engine Air Filters by Ptak, Richburg and Vasseur of Purolator. Unfortunately the Heavy Duty Filter Manufacturers are the only ones that publish factual information on the life- time efficiency of engine air filters. The best quote I have found comes from a paper presented in 2001 "Nanofibers in Filtration Applications in Transportation" by employees of Donaldson Company as follows:
"For most engine air filters, the function of the filter and the corresponding engine wear are based largely on the dust cake filtration. The formation of a dust cake on the filter media changes many important factors in the function of a filter media, including filtration efficiency and pore size distribution. Engine wear rate has been shown to be significant during the early portion of an air filter's design life.
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 consequences from inadequate engine protection."
