The issue with a PC fan, is hydrodynamic drag. The motors just aren't built to handle that viscosity.
You could try 12VDC or lower hobby motors, use a ducted fan/impeller blade type, and craft a housing to work as a circulator for the oil, but it doesn't sound like that is what you are going for, is it?
For the issue of cooling the oil when it increases in temperature, would an aeration type setup work? have a pump lift oil vertically into a waterfall setup where ambient air cools the oil?
And for the CPU fan, as far as to keep errors at bay, why not trick the board into thinking the fan is being used with a resistor setup, similar to if you were to use an extra power supply that has Minimum Load Requirement functioning, if you turn it into a bench power supply for electronics testing?
