Sorry to keep sounding negative, but I just can't see it working, I'd love to be proved wrong but I very much doubt you'll get a fan to start up in oil, and if it did there would be so much drag loading it would most likely burn out.
Oil isn't like water, In my work, (motorsport), an engineer had some oil heaters made. They were basically straight versions of a standard kettle element that were inserted into the oil. The idea was they would heat the engine oil before the engines are run. The theory is fine and it would work if the heating element was much longer and coiled around the oil tank but the problem is the oil in contact with the hot area can't remove it's heat quickly enough to the rest of the oil, so it started to boil around the element. Also due to it's viscosity compared to water, (we were using a thin synthetic oil), it can't get away from the hot part either, (wont mix as well as water).
What you are suggesting is probably possible but would require a lot of development and more than just a tube running in the oil at the top, so I doubt it would work well & I'd guess the hot parts like the fets & some coils would simply over heat the oil around them, in turn over heating themselves.
Along with the sealing issues mentioned before, there are other factors to consider, like making sure the oil is inert to high voltages, just about everything will be conductive to some degree if the voltage is high enough. Another factor will be if the mineral oil is hydroscopic, (like some brake fluids)
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