It is seldom that you separate the final drive from the swing arm tube that you don't see a little lube from the drive migrated there. I drove myself batty 20 plus years ago, changing "O" rings and all, but There was always a little bit to gear lube there. It was never enough to lower the level in the drive itself. I figured at least the input shaft would never be rusty. I just wipe a thin layer of rtv on the flange and tighten the 4 bolts. Put 94K on the first one, 88K on the 2nd, 12K on the one I totalled and 50K plus on the last one, and the little burp of gear lube has never caused any harm.