Hydraulic fluid weights 7.2 pounds per gallon. It has a little lower density than water, which has a weight per gallon of 8.2 pounds. Mineral-oil-based hydraulic fluids are frequently used.
Good hydraulic fluids have just the right amount of viscosity to prevent leakage via seals while not being too viscous to make it challenging for the pump to circulate through the system.
They must be thermally stable so that they can function across a wide range of temperature ranges without degrading and must offer enough lubrication to reduce wear. Manufacturers use foam inhibitors to lower the fluid’s surface tension, allowing air bubbles to pop without creating foam. Foam causes wear and decreases the fluid’s capacity to lubricate.