Quick question, what auto transmission fluid should I be using on my TH400? and which is the best or are they all the same?

I believe it should take Dexron II or the equivalent. Some newer ATF's cover a range of applications, but I don't know how well they will work...maybe even better, maybe no different. I've been told if you use Type F ATF, you'll get firmer shifts, but I think you should use what GM specifies.

Dextron, Dextron II, you say tomAto, I say tomater!!
There's no advantage to using any fluid other than Dextron(II), unless you want to go to a synthetic. Then you can get some extra benefits. I use Dextron II in every trans I build at my shop. I will use an additive(Lube Guard w/friction modifiers) in Chryslers, Hondas, Toyotas, but for GMs & Ferds, it's Dextron all the way. Use Dextron and drive with a smile!!
You can use the old style Ferd fluid, and you might feel a little firmer shift, but it's best to do that trick with a fresh unit; one that has not had any fluid in it at all. Once the friction matierial saturates itself with fluid, it's really hard to get all of it out.
btw..... the fancy B&M "Trick Shift" fluid is actually FERD fluid to begin with!!!

I picked up Texron III (approved for H specifications) will this be ok?  

[QUOTE=RHeyboer] I picked up Texron III (approved for H specifications) will this be ok?  [/QUOTE]


"Textron"?!! Is that some new fluid from Texas?!
If you mean Dextron III, then yes, it's fine. Dex. III is just a newer 'formula', engineered for the late model, computer-controlled transmissions. The viscosity is a little thinner(lighter weight), so the fluid will flow better thru the tiny, tiny holes in the separator plate on the valve body. It works just fine in any application. Go for it!
Really, any fluid will work OK, you just don't want to mix a synthetic with a 'mineral' base oil.

Thanks Joel