DFL dynamos.

[Tony Press]

When I fit sealed bearings (several on my Reliant Scimitar) I carefully pop off the seal, remove some of the grease and add some special anti friction metal treatment which is longer lasting than grease and stays put on metal surface  (remember modern parts are usually only expected to have a short life).
Before someone points out that bearings have to have some friction to work just google bearing friction and read up about research into reducing bearing friction etc....very very technical and most is far beyond what my brain can (wants to) cope with.
My alternator belt tensioning arm pulley bearing is an example of a high rotating use where I have done this.
That stuff good for reducing friction/wear on any metal to metal.......now looking forward to the next century wear free celebrations 

Dennis

(remember modern parts are usually only expected to have a short life).

Not anti-friction bearings- newer bearings from a known provider will last a very long time ! 

Because they are rolling ball bearings  there is very little friction to overcome (which is why they are called anti-friction) so why do this? 

Plain bush bearings are another matter altogether.