Full manual Valve Body, and Diff. Questions

I know a FMVB can go bad, and if it does, will the tranny shift on it's own, or what? I'm thinking about putting the Turbo Action 17675 model in my 727(anybody have their thoughts on the product)

And I've got a Dana Sure-Grip Posi rear end, the clutch version NOT the cone version. What type of fluid should be used for this?(Yes, I know it's a really stupid question to be asked, but any help would be appriciated)