| The incoming air should enter the foam side with the larger size cells. This way the fine cell foam collects smaller particles that the larger cells missed.
Use a filter oil.
The sticky oil is suspended in the path of the dirty air on the strands of the web-like cell structure of the foam. This makes it difficult for small dirt particles to pass through the depth of the filter without sticking to the strands. Larger particles are trapped if they are bigger than the distance between the cell strands themselves.
As the outer wetted strands become loaded with dirt particles and no longer sticky, the wetted strands downstream continue trapping dirt, until the entire foam thickness is utilized. Also, as dirt particles build up on the strands the space between strands decreases, further increasing the filter efficiency by trapping the smaller dirt particles that initially could pass between the strands in a clean filter.
Finally, when the filter is sufficiently dirty to stop trapping small particles and clogging reduces airflow, it can conveniently be washed, re-oiled, and re-used. When oiling a foam air filter, use as little as possible, but do get adequate coverage.  | 
Shazaam!
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