Considering that the cleanliness level of fuel can vary from the point of delivery through storage to its final use in engines (via injectors), engine manufacturers consider the contamination level of incoming fuel to be 22/21/17 according to ISO 4406:99.

  Depending on the injection systems, the fuel cleanliness level must be a maximum of 18/16/13 according to ISO 4406:99 for UEI systems and 12/9/7 according to ISO 4406:99 for CRDI/HPCR injection systems.