I agree with Kevin, there is a felt washer that needs to be renewed periodically. However, the seal I found most helpful to renew was the output shaft seal that rubs on the "distance piece". The "distance piece" then rubs on the felt washer. Kevin is correct that when these two seals are fresh, the 00 grease will stay in the box.
Snidal recommended gearoil, but I found that gear oil leaked at an alarming rate. I chose to use 00 veedol (snapper 00 grease) and it has worked for over 26000 miles.
And speaking of 26000 miles, that is without any rebuilds of the rings, piston, valves, or gearbox. My '99 has been very good to me.