I do not know what will happen running grease in the lower unit. I just know that gear lube/oil should be used.
I don't think I would not like it one bit if my greased lower unit shelled out way down the river in the middle of nowhere.
What will it do?? It will destroy the pinion bearing for one thing. And it isn't replaceable. Once that bearing is gone, so are all the gears and the rest of it.