A gallon of hydraulic fluid weighs 7.2 pounds. It is slightly less dense than water, which weighs 8.2 pounds per gallon. Common hydraulic fluids are mineral oil based.
Good hydraulic fluids provide enough viscosity to prevent leaking through seals but are not so viscous that they become difficult for the pump to move through the system. They must have the thermal stability to operate over a wide variety of temperature ranges without breaking down while providing adequate lubrication to minimize wear. Manufacturers add foam inhibitors that reduce the surface tension of the fluid so that air bubbles break up without causing foam. Foam increases wear and reduces the ability of the fluid to provide lubrication.