Changing hydraulic fluid to ATF

Caldwell Idaho
So even a system that never sees rain can collect water?
Yes, I used to work on forklifts and some older ones would get pitting in the hydraulic cylinders when the owners never changed the hydraulic fluid. The owners would have to buy new cylinders because the pitting would tear up the seals.
Greenville SC
So even a system that never sees rain can collect water?
As others say, YES. Here's a little physics for you.

There is moisture in the air. The air is the reservoir is about oil temperature. The top of the reservoir is about room temperature. The moisture condenses on the inside of the top of the reservoir. The water drops into the oil and settles in the bottom. Cylinders move exchanging air in the reservoir. More moisture. More water.

The company I once worked for built hydraulic power units. Some of our customers specified stainless steel tops on their reservoirs. There was still condensation and water, but no rust particles from the falling "stalactites" of rust.

There are solutions to help this (sometimes work) with desiccant breathers with usually WAY too little capacity.

There are what I'll call air bags which expand and contract sealing the reservoir. This concept was (AFAIK) pioneered by the Greer Accumulator company as Kleen-Vent. I don't know if Parker, who now own Greer, still offer them. IMO, that's the best way, but not always practical, rarely inexpensive.

At least 1 injection molding machine manufacturer incorporates them in some of their machines. There is an open (flattened expanded metal, usually) box with what looks like a bag made of Harbor Freight tarp material inside.
Some older type power transformers had that problem (water ingress) and they use desiccators in their breathers. Now they make them with sealed nitrogen bags, inside the oil.

At 40 degrees Celsius, AW 46's viscosity is 46 cSt, giving it its name. At 100 degrees Celsius, its viscosity thins out to 6.8 cSt.
Valvoline MaxLife, as many ULV transmission fluids, is lower than that, 28.8 cSt (40C) and 5.9 CSt (100C).
I doubt that your hydraulic oil gets to 100C, so the important number is the 40C one, and over there the synthetic is just too thin.
Even the "normal" ATF gets only like 35 cSt at 40C (and about 7-7.5 cSt at 100C).
Plus the ATF doesn't have enough Zinc.

They make synthetic AW64, here is an example of "Arctic grade":
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