Here you have some informations from wellknown companies or institutes, that show that even the smaller particles are critical for engines.
OIL – HOW CLEAN DOES IT HAVE TO BE?
by: Mike Sondalini in collaboration with Don Irvine of Donamar Filters. Data supplied by Donamar Filters
Clean, dry oil can extend equipment life between failure up to 8 - 10 times the normal operating life. Timken, the bearing manufacturer, reports that reducing water levels from 100 ppm (parts per million) to 25 ppm increases bearing life 2 times. British hydraulics research indicate that if solids contamination with particles larger than 5 micron (0.005 mm or 0.0002”) is reduced from the range of 5,000 – 10,000 particles per milliliter of oil to 160 – 320 particles, the machine life is increased 5 times.
If you want extremely low wear rates and long equipment life the evidence indicates that oil needs to be filtered down to sub 5 micron size and preferably down to one micron size. Care needs to be taken that the filter does also not remove any solid additives, such as graphite, in the oil. Additives dissolved in the oil will not be removed unless the additive is attached to a solid particle.
SAE Technical Paper 942032: “It’s a commonly known fact that lubrication oil does not wear out mechanically, and if kept clean will maintain its lubrication qualities intact almost indefinitely”
Contaminants between 2 and 20 microns in size can be particularly damaging since they can disrupt hydrodynamic lubrication by entering wear surface.”
One such study by General Motors concluded that, "controlling particles in the 3 micron to 10 micron range had the greatest impact on wear rates and that engine wear rates correlated directly to the dust concentration levels in the sump."
More information about the book "How to Select a Motor Oil and Filter for Your Car or Truck"