Nice topic - just stumbled upon it. Never tried it, but will definitely do. Got a noisy candidate in my son's computer 
As an ex telecom engineer I must say that graphite does conduct electricity but has a relatively high resistance. In fact old resistors were made by applying a thin layer of graphite on top of a ceramic tube. You will, probably, not short the fan, but I'd be careful to prevent leaks on any bare contacts on the fan/mobo/etc.
On a different note, do you notice any reduction in the speed of the fans? Graphite oil was widely used in electrical and electronic applications, but it has relatively high viscosity. Also, you say that you tried different oils, but they never lasted long enough. I gotta try the oil for sewing machines. It is specifically developed for lubricating the bearings with minimum resistance.
In any case, thanks for the advice!
As an ex telecom engineer I must say that graphite does conduct electricity but has a relatively high resistance. In fact old resistors were made by applying a thin layer of graphite on top of a ceramic tube. You will, probably, not short the fan, but I'd be careful to prevent leaks on any bare contacts on the fan/mobo/etc.
On a different note, do you notice any reduction in the speed of the fans? Graphite oil was widely used in electrical and electronic applications, but it has relatively high viscosity. Also, you say that you tried different oils, but they never lasted long enough. I gotta try the oil for sewing machines. It is specifically developed for lubricating the bearings with minimum resistance.
In any case, thanks for the advice!