Flush system and change "milky" oil....
Heavy equipment like dozers, excavators, logging equipment etc, are a different ball park, and I have 25 years of experience from 10 units of logging equipment and never changed hydraulic oil on schedule reason only (once a year), main reason for change has been WATER. Milky oil in the whole system, 15-30 usg in tank, and 50% out in the system, 10+ actuators and hoses. There is a special procedure to flush the whole system that is that complex. A loggsplitter is easier.
1.extract cylinder, fully out
2.turn of pump
3.Drain tank
4.fill new oil, just little more than enough for a stroke on cyl (2usg?)
5.Pull valve lever for detract cylinder,
6.turn pump (crank w starter or pull coil with ignition off or plug disabled), without starting engine, till piston is fully in
7. (2.) start over from #2 and repeat, but extract cylinder
8.check oil when draining tank, repeat from#2 if there still is "milk" in oil
ps. use the cheapest engine or transmission oil you can find for flushing, if you cant find cheaper hydraulic oil
ps2. for you guys that have a crescent wrench and a big bucket, deattach the return line from tank, plug tank if lower than oil level, and aim into the bucket..