From the book "How to Select a Motor Oil for Your Car or Truck":
Sooner or later, wear from abrasive particles and deposits from carbon and oxide insolubles will interfere with efficient combustion in an engine. Valve train wear (cams, valve guides, etc.) can impact timing and valve movement. Wear of rings, pistons and cylinder walls influences volumetric compression efficiency and combustion blow-by resulting in power loss. Particle-induced wear is greatest when the particle sizes are in the same range as the oil film thickness.
For diesel and gasoline engines, there is a surprising number of laboratory and field studies that report the need to control particles below 10 microns. 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."
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