Hydraulics:

Several machines and processes use a fluid for building a force to move or carry an object, or to control an action. A term hydraulic refers to a liquid. A number of fluids could be used for developing the force. Within a hydraulic system, water, oil, or other liquids could be used. Oil is the most general.

Introduction

Although any liquid can be used in a hydraulic system, as few liquids have benefits over others. Oil is a liquid often preferred as the working fluid. Oil helps to lubricate the several sliding parts, avoid rust, and is readily available. For practical reasons, oil does not change its volume in the hydraulic system while the pressure is changed.