Hey question for you guys, I got a gear diff I'm rebuilding what would you recommend replacing the oil with in it?

Also im pretty positive it needs to be replaced because when you spin the tires opposite directions it feels like there is nothing there... wouldn't that give me a sign it needs to be replaced.

So if I do need to replace it what should I replace it with? Got told 30,000 but just wanted to make sure that's about right.. I just have a 17.5 sensored motor..

Also when I give it throttle it like stutters then goes... But if the diff needs new oil wouldn't that be why.