They generally use grease just like any other prelube / sealed for life bearing that uses balls or rollers does. If anyone does make them with plain bearings things could be as Michael suggests but could also be greased and sealed.
If I was the OP I would follow don't fix it unless it's broke. They are usually well sealed and if the seal has perished, been prised off and damaged etc it's time for some new ones. Same if they feel a bit rough when rotated.
The bearing insert od's in them are often spherical to take care of shaft alignment so they generally last a long time but changing inserts may prove difficult.
Forgot to mention that they may develop end float in the bearings, That can mean replacing them as well.
John
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Edited By Ajohnw on 03/04/2016 11:31:07