Diffusion Pump Oil IOTA704 is a colorless transparent oily, single-component fluid designed for high vacuum of 10-7torr (untrapped). It performs well in tough, rugged applications and offers quick pump down, even after exposure to air at operating temperatures. IOTA704 is thermally and chemically stable allowing it to keep pump jets free of tars and hard carbon.
Silicone Diffusion Pump Oil IOTA 704-IOTA (phenylsilicone.com) 
