What are the main functions for a hydraulic lubricant?

The main function of the fluid in a hydraulic system is to transmit a force applied at one point to another point. The effort must be transferable quickly and with precision.
All quality hydraulic fluids must be capable of fulfilling the following functions:

• Transmit power: transfer of a kinetic or potential energy (movement or pressure)
• Lubricate and protect: protection against wear on surfaces in contact

In order to best fulfil these functions, a hydraulic fluid must have a certain number of properties, right viscosimetry, shear stability, fluid behaviour.