What you oil it with isn't too critical. Pretty much any light oil w
ill do. Plain old 3-IN-ONE or sewing machine oil from the supermarket is fine. Note that the usual red-label 3-IN-ONE that I use has a vegetable oil base, and may gum things up if used over and over. This isn't likely to be a big problem with fan bearings (it's usually only an issue in automotive and other high temperature applications), but if you're concerned then you should get the blue-label mineral based version.
Spray-can oils with an applicator tube can be handy for oiling fans in awkward spots, if you can avoid spraying oil all over the place.
Do not, however, use WD-40 or similar products. WD-40 is has great
penetration, displaces water well, and loosens rust - but it's a lousy lubricant. It's mainly kerosene, and the oil that's left when the kerosene evaporates may be more than adequate for stopping a gate-hinge from squeaking, but won't last long in a fan bearing.
