Choosing a quenching medium for a specific steel alloy, such as the modified 9Cr-1Mo steel (also known as P91 or Grade 91 steel) you mentioned, depends on the desired hardness, toughness, and dimensional stability, along with the specific geometry of the part.

P91 steel is a high-strength, low-alloy steel (HSLA) that exhibits creep resistance at high temperatures and is primarily used in power plants and high-temperature applications. Given its alloying elements, this steel is usually quenched in oil or polymer quenchants to balance hardness and toughness.

While I can't recommend a specific brand or type of oil, the typical quenching oils used in industry are categorised as follows:

P91 is a high-hardenability steel, so a medium to slow quenching oil may be appropriate. Some examples include Chevron Quenching Oil 70 (medium) and Houghton Quench K (slow).

Alternatively, polymer quenchants can offer more control over cooling rates and are generally safer to use, but they may require more maintenance due to microbial growth and evaporation.

Before choosing a quenching medium, please consider consulting with the material supplier, the heat treatment provider, or an experienced metallurgist. Also, remember to take safety measures while dealing with hot materials and flammable oils.

Thank you Mr. Depiver for the information. It will help me to finalize the oil.