"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.
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.