All ways had oil loss from G/box but never serious as it only ever took 1/4 to 1/2 cup to top up, so silver soldered a length of Bowden cable to end of dip stick (60 mm approx I think)to bottom of box, checking level it is always 1/16"to 1/8" below d/stick end. Changed oil mid 80's to synthetic Mobil SHC 130 (or 30, don't recall) as so impressed with results used in worm & crown wheel drives of Purifiers (Centrifuges) from an old Mobil gear oil where we changed steel worm shaft and bronze crown wheel every 4 to 6 month continual operation synthetic SHC would go two years or more changing oil every six months when one saw it with bronze colour. Virtually transparent.
Changed G/B bearings ever time changed B/E up till going to sea and that was three times in four years (Alpha crap} till Proff Higgins found problem with B/E cages on the slant!). 300k since and changed 2010 after 200k, never changed a cog / gear bushes once. With an industrial Dremal put positive rake on gear dogs once so they hold into gear.
Use floppy end of d/stick to check oil level in Altons have three on various bikes with double speed gearing, never a failing of gear or windings as use most of all power generated. Told Alton boys Herve & Paul didn't like their grease system which has failed them miserably, now full circle with off set windings and straight through drive.
Only faith I have is experience.
banana man.