Underfill it a little when the gear oil is cold. Then, check the height of the gear oil after a long enough ride to heat up the gear oil. Then, top-up the oil to fill to the height described in manuals.
Leaks are common - put a disposable diaper on the floor in appropriate locations; AND, - check the fluid/oil levels with a regularity appropriate to the drippage rate.
The common old saying "if it ain't leaking, it ain't got no oil in it" was (and often is now) no joke.
