I've never looked closely at the bearings in one of these but I think I'd guess
them to be the "oilite" type used in many small electric motors (e.g. car
heater fans). This is very often a sintered bronze "ball" in a self-aligning
housing: cheap, robust, reliable. The bronze on steel combination is a good
bearing to start with, and being porous it's helped along by being impregnated
with mineral oil. However, they can dry out (not helped by being in a draught).
There's a trade-off in lubricating them: use something low viscosity and it
soaks in better, but will be more volatile so won't last so long. Higher
visosity oils won't soak in so well but won't evaporate so quickly either. With
WD40 you have a thick mineral oil dissolved in a thinner one: should last quite
well as long as it isn't too warm. Or you could try a silicone oil: a less good
lubricant for metals, but will stand higher temperatures. The excellent
screwfix spray sold for plumbing comes to mind although I have never tried it
on a metal bearing.
My credentials: I've been using combis for > 10 years without a problem in fans
(but have been a specialist in oily bearings for the best part of 35 years).