Requirements are specified in SAE AS4059A which is adopted as a de facto ISO international Standard. For the purposes of international standardization, the outlined modification shall apply to the specific clause of AS4059A.
ISO 11218:1993 history
2017ISO 11218:2017 Aerospace - Cleanliness classification for hydraulic fluids
1993ISO 11218:1993 Aerospace; cleanliness classification for hydraulic fluids