Comparative performances of Litton C, Octoil, Narcoil, and two typical silicones in a non‐fractionating diffusion pump are presented with a discussion of vapor pressure, stability to heat and oxidation, and the ability to operate against high forepressures. Extrapolated vapor pressure, temperature data, and test results show the high boiling silicone to produce the highest vacuum as indicated by an untrapped ionization gauge. The silicon was further found to be nearly completely resistant to oxidation when exposed to air while hot. A table showing relative merits of various oils, as applied to vacuum pumping conditions ordinarily encountered, is presented.