I should have been more specific and said "brake fluid", this is definitely miscible with water, unless the formula is now different from when I was in the motor trade.
From Wiki:
"Brake fluid is a subtype of hydraulic fluid with high boiling point, both when new (specified by the equilibrium boiling point) and after absorption of water vapor (specified by wet boiling point). Under the heat of braking, both free water and water vapor in a braking system can boil into a compressible vapor, resulting in brake failure.[4] Glycol-ether based fluids are hygroscopic, and absorbed moisture will greatly reduce the boiling point over time. Mineral oil and silicone based fluids are not hygroscopic."
EDIT
Explains why changing the brake fluid is a long time scale service attention.
My avatar is a poor likeness.