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[. Full-flow filters, due to their need for low-pressure drop full-flow operation and long life, generally have only a 50% removal efficiency at 5 microns.
OMC Mustang skid steer loaders , in 1979, had a 4 micron hydraulic oil filter on their hydrostatic transmission. They had 40 micron suction filter between the reservoir and the charge pump. Very high pressure and much greater flow than in virtually any engine other than monsters. It is really a matter of the surface area of the media being sufficient to pass the requisite amount of oil. For high flows and finer filtration, you need a bigger can holding lots more media
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Even diesel fuel filters are down around the 6-7 micron range, but the vast majority of cellulose based oil filters are rated somewhere in the range of 18-35 micron @ 50%
It's a trade off between pressure drop, efficiency and above all, cost. The synthetic medias, however, can flow better than the coarse cellulose media with the filtration ability of a hydraulic filter.
Interstingly, they also have much less primary face area than a conventional media, relying mostly on depth filtration (Ben will be along in a sec to correct my terms, I'm sure)
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Rick - what you say is correct, except that almost all automotive filters are depth filters. Depth filters also need a lot of surface area - hence the reason that most filters are pleated. However all depth filters will eventually become surface filters as they load (this is a complex issue...).
Hydraulic filters can be designed to have a lot more pressure drop across the filter than engine oil filters.
Also, it was a common belief (until recently) that you only needed to bother removing particles that were around the size of the smallest tolerance in the engine and larger.
But price is the main factor. Cellulose fibres are very cheap, but far from optimal. Where the customer needs something better (and is willing to pay), then it is of course possible to make something better.
Making the filter larger is not always a good idea. Oil filters work mainly by inertial collection. Increasing the surface area of the filter decreases the average velocity of oil through the media, which decreases the inertia of a given size of particle, making it less likely to be collected.
