Assuming you have a submersible pump of standard vertical shaft design and not a fishtank or pond-pump! Submersible pumps are usualy cooled by the surrounding water, I have never seen one totaly full of oil that depends on it just for cooling and sealing purposes, but just because I havent seen one doesnt mean they dont exist! Again assuming its a standard design sub`pump I believe the oil is there to lubricate and cool the mechanical seals. Most designs of submersible pump have an oil chamber at the bottom of the pump between the stator winding and volute that the shaft passes through, there are 2 seals on the shaft, the top one prevents oil getting into the stator. It could be an ordinary lip-seal or a light duty mechanical seal, the other being the main mechanical-seal behind the impeller. Flygt and KSB are the 2 most popular makes we see although there are others. The amount of oil, its grade and how its filled differ from make and model to model, some have external filler/drain plugs and some have to be dismantled to be drained/filled. We always check with the maker if we can regarding oil grade and quantity, but if its not possible to obtain the information we have always used ISO-32 hydraulic oil as makers always seems to spec` a similar grade and fill the pump with enough to totaly cover the lower mech-seal when the pump is in the vertical.
First thing to do is check if there is a nameplate if there is one and contact the makers or service agents in the hope they will give you the information you require, otherwise you need to involve a pump repair company. Some questions in return...why did you dismantle it in the first place, did it have a problem? Are you sure all the seals and gaskets etc are reusable and undamaged after dismantling it? What is the purpose of the PCB, is it part of a water ingress detection system?
