Earlier quoted context omitted.
Hydraulic brake fluid is glycol ether based and hygroscopic. Planes usually use mineral based fluids which are not, but heck if I know what the F-35 uses.
Quoting ChatGPT (and after a quick sanity check), The F-35 Joint Strike Fighter uses a specialized hydraulic fluid that’s based on a synthetic ester formulation, not a petroleum-based fluid. Specifically, it uses phosphate ester–based fire-resistant hydraulic fluid (commonly in the MIL-PRF-83282 or newer MIL-PRF-87257 class). Apparently the older phosphate-ester based hydraulic fluids were hygroscopic but I'm not sur…
Maybe this: https://en.wikipedia.org/wiki/Tributyl_phosphate
"The major uses of TBP in industry are as a component of aircraft hydraulic fluid, brake fluid, and as a solvent for extraction and purification of rare-earth metals from their ores"
It might be better if it is hygroscopic as the water won't separate and risk forming ice plugs in the hydraulic lines.