P Zero

Ok heres my dilema, I had the trans in my DD rebuilt. Cost me right around $1000. I was going to put AMSoil in it, but the techs I work with STRONGLY recommended that I dont use synthetic. They said that the trans isnt designed to run that type of fluid. And that the amount of slip of the clutched is designed with a specific fluid in mind. And that if I make it more slippery, itll throw it all off and possibly damage my freshly rebuilt trans. Is this true?
I dont know much about automatics so I need some expert help. Preferably someone that remans trannies as their job.
BTW, the trans is a 4-EAT also known as CD4E or GL4AE, or 2 pan ecat.
-P.