Well, good question Greg... I'm assuming they had wheel bearing grease inside them when they were initially put together.
The last time I took them out, the silicone I put on the inside of the bearing trying to seal it to the end stuck the seal itself to the housing and when I pulled the axle, I got to see what the inside of the bearing looked like. I just put my thinking cap on and filled the bearing back up with wheel bearing grease and pressed the seal back into place.
Same thing happened this time only this time the entire bearing was full of gear oil because so much of it was getting past the "seal" and into the bearing. I packed it full of grease again and cleaned all of the silicone off of the seal and put it back in place...again.
The bearing feels great, and it's seen some abuse over the years...but I'll keep an eye on it for sure.
Here's what the axle and bearing look like:
I think I took those pictures the 3rd or 4th time I had them out trying to seal them up.