1. The viscosity is appropriate with minimal variation with temperature changes. A high viscosity working medium leads to significant pressure loss in the system, reduced efficiency, and worsened suction conditions in the pump, making it prone to cavitation and erosion, thus difficult for the pump to operate. On the other hand, a low viscosity working medium results in excessive system leakage, increased volumetric loss, and low system efficiency, also deteriorating the rigidity of the electro-hydraulic actuator. Moreover, as seasons change, and during the start-up and normal operation of the machine, the temperature of the working medium varies. Therefore, to ensure normal and stable operation of the electro-hydraulic actuator, it is required that the viscosity of the working medium changes minimally with temperature.

2. Antioxidation. The working medium undergoes oxidation and deterioration upon contact with air, with factors like high temperature, high pressure, and certain substances such as copper, zinc, and aluminum accelerating the oxidation process. After oxidation, the acid value of the medium increases, its corrosiveness enhances, and the thickened substances formed by oxidation can block the pores of the components, affecting the normal operation of the system. Therefore, it is required that the working medium possesses good antioxidant properties.

3. Prevent Rust and Corrosion of Metals. Many metal components in electro-hydraulic actuators are in prolonged contact with the working medium, causing their surfaces to corrode under the action of moisture and air dissolved in the medium, thereby damaging precision and surface quality. Rust particles circulating within the system also lead to accelerated wear of components and system failures. Additionally, the medium itself must not corrode metal components, as it may slowly decompose to produce corrosive substances like acids. Therefore, the hydraulic working medium must have excellent protective properties, preventing rust and corrosion of metals.

4. Excellent anti-foaming and de-foaming properties. Air mixed into and dissolved in the working medium often precipitates in two forms: bubbles (diameter greater than 1mm) and misty air (diameter less than 0.5mm), which is known as foaming. Foaming media can reduce system pressure, worsen lubrication conditions, decrease operational rigidity, and cause abnormal noise, vibration, and cavitation in the system. Additionally, the large surface area of air bubbles and misty air accelerates oxidation upon contact with the medium, thus requiring the working medium to have good anti-foaming properties.