Motor manufacturers have recommendations which are specific to the device and service category (ambient temperature, load, duty cycle, air quality, dust, particulates, etc.). These vary for journal, roller, and radial ball bearings. In the case of electric motor bearings; cleanliness and free air circulation for cooling are critical. Ball bearing are generally sealed or shielded, and permanently greased. In cases where they are being restored as they approach the end of design basis service life; they can be cleaned in an oil bath (not to exceed 100C), rinsed in solvent, and regreased to ?-? of the free volume. Excess grease is universally regarded as destructive. Over greasing is said by motor manufacturers to create overheating which boils off volatiles and changes the properties of the grease. The vapors from condensation deposit nearby trapping dust, blocking air flow, and exacerbating overheating. Lubricant manufacturers cite degradation of lubricant due to shear.
There's a few different thoughts on greasing motors so its subjective mainly in regard to doing it running or not running. A big problem from overgreasing is if the bearing gets pressurized. This will lead to premature failure. Also extra grease can be forced in among the winding which isn't great. Some motors, especially larger ones, have plates to remove so that you can tell grease has made it through and to prevent pressurization. The manufacturer will give you the best guidance on type and amount of grease, procedures etc for their motors. Also be careful if the motor has been worked on so may not have original grease and some greases don't mix well.
