third member sealant

The Right Stuff is a good sealant and can actually eliminate the paper gasket.
The problem is, The Right Stuff really glues the two parts that it is used on together. It can be a pain getting the parts to separate if you ever need to go back in to what it was used on.
For a differential housing, I make sure the mating surfaces are super clean, use the paper gasket and a light coat of a sealant like Permatex or RTV.
I once assembled a differential housing with just the paper gasket, no sealant. It leaked, not much, a very occasional drip, but that’s no good. So it came back apart, new paper gasket, light coat of sealant and all was good.