Grease is a thick film lubricant for heavy loads that require a high film strength. It may work on old fans where the clearances are high, but will not work efficiently on newer fans with the correct bearings clearances.
As someone already mentioned, sleeve bearings (like 'Oilite' bushings) have impregnated sintered bronze bushings with an open cell structure. Generally, this is impregnated with a lifetime lubricant. However, a drop of oil applied sparingly will help prolong the life, and quieten the bearing. Synthetic oil such as Mobil 1 (SAE 30) will work fine. (BTW, the 'WD' in WD-40 stands for 'Water Dispersant').
Do not use a spout to dispense the oil. Instead, use a piece of bare 12 Ga. or 14 Ga. copper wire, dip the end in the oil, and touch it to the bearing until you get about 1/2 to 1 drop of oil on the bearing. This should last about 6 months in normal use.