was wondering what fluid I could use for a trans dump and fill. I have 75-90 Valvoline synthetic. Can I use this fluid for the diff too?

 

Assuming you're talking about manual trans. Usually, no, the trans fluid are GL-4. But if the bottle you have says its also rated for GL-5 hypoid diffs, then you should be. I'm running amsoil severe gear in my rear of the same viscosity, but run chevron Deleon in the trans. Depends on what the bottle and what your owners manual say... Should hopefully hear what others recommend.