I agree with rgatijnet1, just fill it full. I think on some hub covers (they're made by a lot of different mfgs) that you have the minimum line that you saw and then there's another line that usually isn't marked between that and the fill hole. I believe that's the full mark, but again, I fill mine up to the fill hole.
As long as you don't see any active leaks, 1/8" of difference between the two sides is nothing to worry about - that's probably because the MH isn't exactly level.
As part of your maintenance, the caps should be removed every couple of years and the oil drained. Look at what was drained and inside of the hub for excessive metal or any small hunks.
I carry an extra quart of hub oil with me because I've had one leak and other crack over the years and of course there weren't any new covers within miles.
Yes, the center cap the size of quarter, usually red, but often black, is make of rubber and pulls out.
Bill