The balls ride on an extremely thin layer of grease, as they push the rest to the sides. I can't imagine grease adds any appreciable amount of friction, even the most viscous grease. Just don't worry about it.

Any grease will work fine until it fails. You don't know it has failed or is near failure until you open up the bearing, so overhaul your bearings at reasonable intervals to get there before the grease dries up.