IIRC, for purely 'dumb' voltage controlled fans immersing them in oil is not a problem: they will receive the same power, and just spin more slow.y. But if the fan control uses feedback, then there may be a problem: the fan controller will sense that the fan is spinning far below its target speed, and attempt to keep increasing the voltage until the fan is at the 'correct' speed. Because the oil wills prevent this from happening, the controller will end up just applying the maximum possible voltage all the time. Depending on the assumptions made during controller design, this could result in anything from merely running the fan continuously at its peak rated power, to overloading the fan if the controller fails to properly cap power (e.g. if the controller was designed assuming it would only be compensating for small fan variance).