Sorry if it was here before, but which oil do you use? Because in the handbook, there is specification GL4, but it's not the right oil for hypoid gears, what final gear really is, so it have to be filled with GL5...
The best is to use a synthetic transmission oil, which will give you smooth changes even when cold and better protection generally, just like synthetic engine oil is best for engines.
I've always used Mobil SHC 75W/90 but any good quality brand should be fine.
Roy