Originally Posted by DS Motorsport

Use gasket sealant.
I use Loctite 5926, it's even reusable.
On one of my diff's i applied the sealant in march when i rebuilded the car.
In the meantime i changed the diff oil about 3-5 times and the original sealant is still on the diff and the diff's never leak a single drop.
It also saves you a lot of time compared to the o-ring.