What a fan bearing actually is
Most household fans do not have ball bearings. They have a bush made of sintered bronze — bronze powder pressed and baked into a porous solid, then vacuum-impregnated with oil. The oil lives inside thousands of microscopic pores and seeps out slowly as the shaft turns, which is why the part is described as self-lubricating and why it is expected to last years without attention.
Now the part that matters. Because the oil is inside the metal, oil applied from outside cannot get in. A drop on the shaft sits on the surface, and when the fan runs, that surface is the narrow gap between shaft and bush — the one place where dust is already accumulating. Oil plus dust is a grinding paste. So the well-meant drop of engine oil before storage does not lubricate the bearing: it creates an abrasive and holds it exactly where the wear happens. A summer of running, and the bush is a little bigger, the shaft a little looser, and next June the fan hisses.
Which gives you a clean way to read the fault. Guard off, blades off, power off, and turn the shaft by hand. If it is stiff, gritty, or turns a few degrees and stops, the bush is the problem — and the repair is cleaning, not oiling. If the shaft is silky and free but the whole fan shakes at speed, the fault is balance, and the cause is almost always dust on the blades. If the shaft is free and the fan does not shake but buzzes, the noise is a panel resonating, and your hand on the guard proves it in one second.
Dust on a blade is a balance problem, not a dirt problem
A fan blade is a light plastic part spinning at a thousand revolutions a minute, and the dust that settles on its leading edge does not settle evenly. Even a few grams of difference between blades becomes a force that grows with the square of the speed, which is why the fan is quiet on setting one and tries to walk off the table on setting three. That is the whole fault: not that the fan is dirty, but that it is dirty asymmetrically.
So wash the blades properly — warm water, a soft cloth or brush, and dry them completely before they go back on — and take the opportunity to check that each blade is sitting fully home in its slot on the hub. A blade that has not clicked into place is out of position by a millimetre, and that is a balance fault you cannot wash away. If the shaking persists with clean blades correctly seated, the blades are old and one has deformed; a set of blades is inexpensive and it is the only real cure.
Why the plastic bag is worse than no bag at all
Everyone stores a fan the same way: wipe it, bag it, put it in the loft. The bag is the problem. A sealed plastic bag traps the air that was in it on the day you closed it, and that air carries its own humidity. When the temperature drops overnight the water condenses on the inside of the plastic — you can see the droplets if you look at a bagged fan in November — and it stays there, because nothing can evaporate out of a sealed bag. Six months later the bush has been sitting in a damp microclimate, and the rust and the staining you find are not from the loft, they are from the bag.
What works is the opposite of airtight: clean, dry completely, cover with something breathable — a cotton sheet, a pillowcase, a proper fabric cover — and stand it upright. Upright matters for a plain reason: the bush carries the rotor's weight, and stored on its side for six months the shaft rests on one line of the bush instead of being held evenly round it, which is where a flat spot comes from. Store it somewhere dry, not a damp cellar and not against an outside wall, and next summer the fan you put away will be the fan you get back.