Disclaimer: I am not engineer so you may find some BS on the text below
I think that you miss something when using grease on a gear diff. Oil gets kinda stiffer as the diff works harder and friction increases (this is what gives pack to the shocks), but grease stays more or less the same. You get too much diffing out and you get a bad handling car with grease on the diff, as well as too thin of an oil.
Another issue is that the Kyosho gear diff is not very well made. I have two of them and they leak like crazy! So I used some some black gasket sealant and the leaking is gone. Not very handy but Id' rather mess with sealant than get oil everywhere on the gear box but on the diff itself.
Edit: I didn't knew there were CST rated greases. I was only commenting on "normal" greases such as graphite grease.