Originally Posted by
noglider
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.
In addition to that, the torque that wheels put on hubs - I believe wheel that are out of true slow the bike down a lot more than the worst friction in the hub bearings can.
Regular, car, lithium grease, with NLGI 2 thickness is as good as it gets IMO. Going more expensive than that doesn't make any difference I could notice - both performance and maintenance/durability wise. Just make sure ball bearings are good (or new) and it's all cleaned well... and put as much as possible - wiping off the excess that gets pushed out - "if in doubt, add more" is my rule.
The best bicycle bearing grease