Sleeve bearing fans seem to get a bad rap, but I have a house full of room fans, and find even when new they have very little lubricant; but I pull them apart, add oil, and find they now run indefinitely. I have one that runs 8-12 hours a day and is still doing fine after more than 10 years; then again I do run it slow on a variac.

One trick to check the bearings is to rotate the fan and see that the fan blades do not follow.

I have also seen no failures in my sleeve bearing computer fans after I have them well oiled.