polyurea greases are often used in applications where very long grease life (up to fill for life) is required. another consideration is the amount of noise generated by the bearings - with some polyurea greses the noise emissions are far less then with others. based on these arguments the grease was most likely carefully chosen. although other greases may well be compatible, mixing may lead to various unwanted effects that only come to light after very long use. you are better of to stick to the grease advised in this case and only consider a changeover when changing the motorbearings - and then choose a suitable grease together with your lubesupplier, the bearing manufacturer and the motor manufacturer.
if for one reason or another you need to regrease the bearings frequently, my advice would be to pump out as much of the original grease as possible the first time you do so, to avoid mixing greases as far as possible.
compatibilitycharts on the subject of grease mixing should be read with care: there is a large difference between what can be acceptable in the case of chassis lubrication of a shovel (frequent relubrication, after a few times all the original grease is gone) and when regreasing electric motor bearings where only a few shots of grease may be needed every 2000 hrs or so and where thus various greases may be in intimate contact with each other over a very long period.
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