I have some tips for the metall gears.
Please try to use only two of the small gears instead of four. This will make the diff more smooth. Just go up with the oil, around 2000-3000cst.
After the first assembling, run in the diff, open it again, clean it with brake cleaner and refill it.
This will help to get all the dust and maybe some small metall parts from production out.
Freddy