Hydraulic oil is often contaminated even when new. In this case the new hydraulic oil was fairly clean with a starting ISO particle count of 17/16/13.
We let the CJC kidney loop filter run overnight to see how clean we could get the hydraulic oil. After 14 hours of continuous filtration the final ISO reading was 10/8/5 . The owner of the drill was happy with the results and had never seen the oil that clean in the drill hydraulics.
Location: Coal Mine in Alberta, Canada.