Originally Posted by DS Motorsport

Personally I don't bother using the O-rings, they can cause a leakage or need regular replacement.

I make my own O-ring using liquid gasket, liquid sealant.
Basically I fill up the area where the O-ring normally sits with liquid sealant (Loctite 598).
First clean the diff housing very well, you want the gasket to stick to the housing.
The cap, or flange you want to leave a bit oiled.
This way the gasket doesn't stick to the flange and can be left in place.
I've opened up diffs 10+ times without replacing the liquid gasket.