I never had much luck fixing/cleaning the bearings of such fans.
A clean/re-oil usually lasted about two weeks.
If the bearing/fan got stuck, after a while the thermal fuse would let go.
Those bearings are bush-type synthered bronze bearings, soaked with oil.
Just adding extra oil does not seem to work.
May be if you would boil the bearing in oil it would help. Or just get new bearings.
But then it gets to the point where it is simpler/cheaper to get a new fan, at the end that is what I did.
EDIT:
I did try grease as well, did not really help either.
Good luck, Peter