From my experience, I would stick with the ATF+3. I ran the Amsoil +4 for a little over a year and felt it was almost too slick. I followed what DTT said and put in ATF+3 and by does my transmission feel better when it shifts. Don't forget too that ATF+3 is about 1/4 of the price as the Amsoil stuff. Yes the Amsoil fluid is top quality, but as Katdiesel said, every truck will run different.
Kev