Leaking rear diff after service

[Vash]

As long as the mating surface is level, the o-ring should be enough. That assumes it's seated in the channel properly. I put a little diff oil on the o-ring similar to an oil filter seal. Additional sealant won't hurt. Regardless, sealant should be used on the threads of each bolt. Although, I'd use a black or grey RTV type sealant. In particular, I like Permatex Great Stuff.