The working medium used in hydraulic presses serves not only to transmit pressure but also to ensure the sensitive, reliable, and long-lasting operation of the machine's components with minimal leakage. The basic requirements for the working medium in a hydraulic press are:

① Possess suitable fluidity and low compressibility to improve transmission efficiency;
② Provide anti-corrosion protection;
③ Have good lubricating properties;
④ Be easy to seal;
⑤ Maintain stable performance without deterioration over prolonged operation.

Hydraulic presses initially used water as the working medium. Later, this was replaced by an emulsion formed by adding a small amount of emulsified oil to water, aimed at increasing lubricity and reducing corrosion. In the late 19th century, oil hydraulic presses using mineral oil as the working medium emerged. Oil offers good lubricity, corrosion resistance, and moderate viscosity, which contribute to improving the performance of hydraulic presses. In the second half of the 20th century, a new type of water-based emulsion appeared. Its emulsified form is "oil-in-water" rather than the original "water-in-oil". The external phase of the "oil-in-water" emulsion is oil, giving it lubricity and corrosion protection close to that of oil, while containing very little oil and being less flammable. However, the higher cost of water-based emulsions has limited their widespread adoption.