Inside of many submersible electric motors you will find a very thin, clear oil. It is very easy to mistake it for water. But don't dump it out!
Dielectric Oil is a thin, light, clear, highly-refined version of mineral oil. It is non-conductive, so it is OK to be inside the motor. Standard mineral oil cannot be used as a replacement.
It is placed inside the motor as a cooling agent. The oil absorbs the heat produced by the motor and transfers it to the outer shell of the motor. The cool water on the outside of the submersible motor can then absorb the heat and cool the motor.
If you did, by mistake, dump out the oil from your motor thinking it was water, you will need to replace the oil before operating the motor. Because we buy the oil by the tanker load, we do not sell it as a replacement product. If you need to purchase some, you could check your area for an electric motor repair shop. Ask if they work on submersible electric motors. If they do, they probably have the oil You can also find the oil online at vaious places. A simple internet search should turn up vendors.
If you have concerns about the health and safety properties of this oil, you can download the Material Safety Data Sheet (MSDS) here
And you can see a Wikipedia article on this oil here