Ford's answers to the NHTSA 6.7 Investigation

This study is very dated, 2007ish, but it contains an interesting and possibly relevant quote --- that applies to diesel injection systems circa 2007 (and not later).

Chevron Diesel wrote:
ASTM D 975 limits the ash content of most diesel fuels to a maximum of 100 ppm. (Inorganic particles and oil-soluble, metallo-organic compounds both contribute to the
ash content; but, only inorganic particles will cause wear.) The U.S. government has a tighter speci?cation of 10 mg/L (approximately 12 ppm) for all particulate matter. However, neither speci?cation addresses particle size. While most fuel ?lters recommended by engine manufacturers have a nominal pore size of 10 microns, 6 studies by the Southwest Research Institute reveal that the critical particle size for initiating signi?cant abrasive wear in rotary
injection fuel pumps and in high-pressure fuel injection systems is fr
om six to seven microns.

However, as engine designs to reduce emissions result in higher fuel rail and injector pressures,the tighter clearances will have less tolerance for solids and impurities in the fuel. Consequently,
some engine manufacturers are now specifying ?lters with pore size as low as two microns.

Chevron: Diesel Fuel Technical Review 2007