Sounds similar to DOT-5 brake fluid. Maybe this: https://en.wikipedia.org/wiki/Tributyl_phosphate https://en.wikipedia.org/wiki/Tributyl_phosphate "The major

Sounds similar to DOT-5 brake fluid. Maybe this: https://en.wikipedia.org/wiki/Tributyl_phosphate 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.