agree its a level /filler
best done parked on the level ground, access can be a fiddle , easy if the tunnel is off
many make a access opening in the tunnel so checking is made easier
the input shaft is quite likely a scroll not an oil seal so parked on inclines can show up as a leak
when not running , as needs to rotate to spin the oil back into the case .
the advantage is ,,,,,it cant wear out .
the diff also has a level/filler located just behind the N/s drive output shaft
best accessed by removing the rear wheel and look through the chassis.
Pete