PURPOSE: To improve rust prevention and oil/water separation by incorporating a given amount of a neutral calcium (barium) sulfonate into a rotary pump oil comprising as the base an alkyldiphenyl ether excellent in oil/water separation.

CONSTITUTION: At least either of a neutral calcium sulfonate represented by, e.g. formula II or III (wherein R is an 8-20C alkyl) and a neutral barium sulfonate (represented by, e.g. a formula which is the same as formula II or III except that the Ca has been replaced by Ba) is added to an alkyldiphenyl ether having a kinematic viscosity at 100°C of 2.0-30.0mm2/s and represented by formula I (wherein (m) is 1-3, (n) is 0-3, and R is a 10-20C alkyl, provided that when there are two or more R groups, they may be the same or different) in an amount of 4.0wt.% or less, preferably 0.05-3.5wt.%, based on the total amount. In addition to the neutral salt, optional additives such as an antioxidant, antiwearing agent, metal inactivator, rust preventive, and antifoaming agent may be used for respective purposes.