I know this will sound like heresy to many, but I never will be as concerned about engine air quality as much as oil and gas quality. I use Amsoil oil filters and I feel that is more important than whatever oil I use- Amsoil or whatever. I researched oil filters for a long time on the internet. Amsoil seems far and away the best, which is important since so many oil filters are plain garbage internally.
So long as the air filter is passing air and looks recognizable, the total potential weight of air impurities seems much less than that of oil and gas impurities to me (I could be wrong about this), and the exposure to the engine seems very limited by comparison. I would not expect dry air impurities to come into contact with and be retained by engine oil or parts in significant amounts (again, I could be wrong).
For air filters I replace based on visual inspection, and I'm conservative about replacements. So long as at least half the filter is not saturated it's fine by me.
Some people over-react at the first sign of saturation in the valleys and peeks. Look- if the narrow valleys and peeks are not passing air, the airflow will have already taken the next most efficient course one millimeter down or up the fold. The only caveat is that a filter MUST of course use an element which will not pass dust when saturated. If a filter passes dust when saturated it is a serious problem. Filter size and the normal dust level and type of your region is also a consideration. Dust in Nevada must be the worst. It is salty and alkaline. Some large engines also seem to have small filters which would need replacing more often, and be effected by filter efficiency more.
That said, I WOULD pay more for a filter which uses Gore's "clearstream" filter material. After nursing a paper wet/dry vacuum filter after multiple cleanings and wet cleanings (now a wet/dry vac MUST be the most demanding filter application, right?), I jumped at this filter when I saw it at Sears, and it was some of the best money I have spent. It is a white, teflon impregnated plastic sheet fan-folded like paper. It passes air amazingly freely but supposedly filters down to a few microns (the exhaust certainly smells and seems super clean- I would not take issue with the claim). The main advantage besides air capacity and effectiveness is LONGEVITY and DURABILITY. I have cleaned it MANY times. It is plastic, so it is totally unaffected by wet cleaning (or vacuuming water). The cleaning instructions are very specific. DO NOT USE A BRUSH. A brush will normaly permanently embed dust into the filter. Use water and water back flushing only, and only at moderate pressures. This filter becomes TOTALLY saturated and buried with dust so fine it is probably an environmental hazard, but the filter is always as good as new after cleaning. Prefilters are also a good idea, but keep in mind that prefilters have much less surface area than the main filter, and if they are really needed they will saturate far sooner.
I don't yet know if any auto air filters are using a material like this, but I would gladly pay a premium for it if I found it.
[ June 13, 2003, 10:49 PM: Message edited by: Fieldlab ]