Diff oil will leak out of a nitro rustler diff. The diff is not sealed in any way. Silicone diff oil can be used, however it will make somewhat of a mess when it leaks and will require refilling periodically. I've seen higher viscosity diff grease that has weight ratings just like the diff oil does. This would be the alternative to regular automotive grease.

The problem with nitro rustler differentials is from the factory, they come with grease in the diff. Chances are some of the grease liquified and seeped out of the diff. You will want to pull the diff out and look through it. Most likely it needs a fresh load of grease. Axle bearing grease is fine but if the problem persists after re-packing it with grease, you will probably want to find a thicker grease.