Ultra-high vacuum diffusion pump oil is the core working medium for achieving and maintaining an ultra-high vacuum environment. It is not ordinary lubricating oil; instead, it is a special liquid with extremely low saturated vapor pressure, excellent thermal stability, and chemical inertness. Its performance directly determines the ultimate vacuum degree, pumping rate, and system cleanliness of the diffusion pump. 
The current mainstream high-performance products are represented by synthetic silicone oil and polyphenylene ether type oils. Among them, poly methyl phenyl silicone oxide with high phenyl content (commonly known as 275 type silicone oil) is particularly widely used. This type of silicone oil has an extremely low saturated vapor pressure at room temperature and its ultimate vacuum degree can reach the order of 10⁻⁸ Pa, which is a qualitative improvement compared to traditional mineral oil. It possesses excellent "three-resistance" properties: high temperature resistance (flash point is usually above 240°C, and it can work continuously at temperatures above 200°C), low temperature resistance (freezing point is lower than -14°C), and radiation resistance. In addition, its chemical properties are stable, and it is not prone to oxidation and decomposition, thus maintaining its performance for a long time. 
In practical applications, these characteristics have been transformed into key advantages. The low vapor pressure ensures that the oil molecules themselves do not easily volatilize and reflux under high-temperature conditions, preventing contamination of the cavity that requires ultra-high vacuum. The excellent thermal oxidation stability guarantees that the oil does not easily decompose and form residues under the catalysis of high-temperature metals, thereby extending the maintenance cycle and service life of the pump. Therefore, this type of pump oil has become an indispensable consumable in cutting-edge fields such as semiconductor manufacturing (such as ion implantation, coating), high-energy physics research, and aerospace device testing.