dont split it !. the gasket is no longer around !!!. I run same gear oil that goes in the front and rear diff's...80w-90 full syn gear oil. if a gasket was around ? ( they are not ! ), you could just split it, wash it out that way with just about any kind of cleaner, I would use my parts washer in this case. in your case, just drain it, re-fill with fresh 80w-90 gear oil, and call it done .