Replacing tranny and diff fluid

I'd like to replace my 5spd tranny fluid in my '00 GT. I don't know the exact spec on the 5spd, but would like to use Amsoil. What type of fluid do I need to add. Also will the Amsoil synthetic help to make the shifting smoother?

What weight of diff fluid should I use? I also like to use Amsoil for the diff.

Thanks.