Is hydraulic fluid always supposed to be just as clean coming out as it is going in?

[The motor guy]

I just changed the hydro fluid on my gravely mower. This mower has a separate parker pump and wheel motor system. I have to say it sure looks a lot better than the stuff that comes out of these all in one integrated hydro gear transmissions. Anyways, I think I had 600hrs on this change. Recommended interval is 400hrs and this time it was just a tad darker than what new fluid is. Am I wasting my time draining out fluid that still looks brand new? Also, they dont actually call for hydraulic fluid, they call for 20w50 motor oil

 

[dnewton3]

There will be metals (typically Fe) that is in the fluid which would be seen in ICP that's not visible to the naked eye. (Not unlike a typical differential; mostly Fe is the wear item from the ring/pinion gears). But generally, yes, these systems run fairly clean. There's no combustion process to deal with; no burning of hydrocarbons, no moisture as a byproduct, etc. They can get fairly warm; some oxidation will take place.

 

[The motor guy]

There will be metals (typically Fe) that is in the fluid which would be seen in ICP that's not visible to the naked eye. (Not unlike a typical differential; mostly Fe is the wear item from the ring/pinion gears). But generally, yes, these systems run fairly clean. There's no combustion process to deal with; no burning of hydrocarbons, no moisture as a byproduct, etc. They can get fairly warm; some oxidation will take place.

I think shearing is also a concern