which transmission fluid to run/filter kit?

[fatness_42]

I am going to do a trans flush and filter change. I was just wodering what fluid to use and filter kit looking for some feedback. I see a lot of guys that say use the diamond STP II but I want to go full sythentic. I see a lot say Amsoil but I was thinking of going with the castrol +4 full sythentic. Which filter should I run OEM or something from the parts house? I am a DSM owner for a lot of years but I have a new Auto that I just bought. My other is a 5 speed. The auto has just over 69000 miles and I want to do a flush for 2 reasons. One being I find it pointless to just do a drain and filter change when I don't know how the fluid was done before. And I want to use a full sythetic and don't want to mix the 2. Any input would be helpful. Thank you. Also which cooler line is the return line?