Special, stringent care is required when handling aircraft hydraulic fluid, as it is critical to flight safety that it stay free from contamination. It is also necessary to strictly adhere to authorized references when servicing or repairing any aircraft system. Samples from aircraft hydraulic systems are taken during heavy aircraft maintenance checks (primarily C and D checks) to check contamination.
[http://everyspec.com/MIL-STD/MIL-STD-1100-1299/MIL-STD-1246C_NOTICE-1_21759/ Military Spec 1246C] is one fluid contamination specification.
The ISO fluid contamination scale assigns a contamination category based on particle size count and distribution.