The bearings for the gyro are a very small ball race and it would seem that they are happy with the very light oil that is applied at manufacture or overhaul.
I would caution about buying overhauled gyros, most seem to last about half as long as a new unit.
The only exception to this have been the gyros I have had overhauled by Skysmart avionincs three of my aircraft have gyros overhauled by Skysmart and there product seems to last as long at factory new gyros.
Skysmart
I have no interest in Skysmart except as a very happy customer.