Hey Max ... as a point of reference on filtration levels , our gear pump division ( the old Borg Warner ) said that 25 micron was acceptable for what we called sleeve bearing pumps , or as others may call them , plain or journal bearing pumps . The only time we needed to get into super low micron levels ( 3 micron absolute ) was with piston pump equipment . I haven`t followed the current thinking for a while , but do remember some "discussions" between engineers about filtration levels getting so low that there was the possibility of removing some of the hydraulic
oil additives .... but they were talking about 3 microns and finer . Our engines are very similar to hydraulic pump/motors in the bearing concept ... and requirements . I`m very comfortable with 25 micron levels for an engine .
Filtration is somewhat about trade offs . The finer you go , unless you increase the dirt holding capacity ( more filter media area ) the shorter the element life ... to a point . The initial start up is when the system is the dirtiest and when the filter life is the shortest . As the system is run and cleaned up , the element life increases ... until some component starts to fail or contaminant is introduced from outside the engine . As a filter "clogs up" , you do filter smaller particles out , but you also increase pressure drop across the element ... that and also cold start ups , is why there are bypass valves on the filters .
As a side note ... we used to make a lot of roller bearing pumps/motors for the mining industry since filtration/maintenance there was horrible . Testing and practical experience showed that the rollers actually crushed the larger particles into "smaller" ones and the pumps lived longer .... which a plain bearing can`t do .
This is a VERY basic and brief background on a very complex subject that filter companies have spent a lot of time , money and research on over the years .
Thanks for all your work on this subject that a lot of us will .... or should , benefit from !!!
Bob