The seals weren't designed for true oil bath so they wouldn't hold oil for long. I know a lot of guys who will thin out their grease by pouring some 90wt in it and giving it a stir. Works pretty well, especially in REALLY cold weather when hubs tend to stick if you just use straight grease.
When I worked at Freightliner, we had one carrier that spec'd a "special" (read very expensive) grease for all their trailer bearings. It was the most vile disgusting hot pink color in the world and had the consistency of mayonaise, so it was pretty thin. Don't remember who made it, but was approved for disc brake/high temp service and due to it being very thin, it worked really well. Never had another bearing failure after they started using it. I'll look into it and see if I can figure out who made the stuff.
