I want to run an AC motor submerged in hydraulic oil. What do I need to do?
Motors designed for this are sealed, you don't have to do anything. If not sealed and not brushless the brushes will get coated and the motor will not run.
Verify that the pump seals and gaskets of a sumersible motor/pump assembly are rated for the fluid being pumped. You may need to order a unit with teflon vs. rubber seals for example....
