In order for a dry media, paper air filter to filter dirt effectively, the filter and its fibers must be thick and densely compressed in order for them to filtrate up to industry standards. That being said, this denser and thicker media becomes more restrictive, not allowing air to flow as fast and efficiently as it should. Due to this restrictive media, over time, a dry media paper filter will build up with more and more dust and dirt particles.
Once debris has built up to a certain point, the pressure inside the filter will drop while the air pressure outside the filter will remain the same. If the buildup gets too bad, and the difference in pressure becomes too great, it can result in a dry media paper filter to cave in or collapse on itself. In addition, an excessively high difference in pressure between the inside and outside of the filter, brought on by an overly clogged filter media, can literally pull dirt particles through the paper medium. This causes the air flow and filtration to decrease as the filter becomes more and more clogged.
In comparison, oiled filters include multiple layers of oiled cotton or gauze fabric which captures dirt and debris more effectively. The debris entering the filter will actually stick to the oiled fibers of the filter, and actually become part of the filtering media. This process, sometimes referred to as depth loading by some filter manufacturers, allows the filter to retain more dirt per square inch than a paper filter. On many oiled filters, the cotton or gauze fabric is then meshed between pleated aluminum screens. Pleating the layers of filter media increases the surface area which allows the filter to be in use longer, and capture more debris than a standard non oiled and/or non-pleated filter media.
As dust particles enter the filter they are stopped by the interwoven layers and are then captured in place through the oil. Dirt and debris that are retained on the surface of an oiled filter media have little effect on air flow because there are no small holes to clog like there would be on a paper filter media due to the interwoven layers. So when a paper filter starts to clog, an oiled filter media is still filtering debris just as effectively as when you purchased it, as well as retaining a higher level of airflow. This is due largely to the oiled filter media as well as the method that is used to hold the layers together.
