When my car spent 2 months in the shop for a "no-start" problem (caused by mother nature) , I was very fortunate to have an excellent Crossfire technician.
He changed the ATF+4 that came from the factory with synthetic motor oil. As MAXWELL pointed out, I think it was 0w40.
I had picked up 2 different kinds of "super slick" transmission additives from Summit Racing, and asked him to add it to the tranny. He told me he would if I really wanted it, but he said, there really isn't anything out there that's more slippery than synthetic motor oil. It made sence to me, so I took his advice.
I really can't say I noticed any real difference between the motor oil compared to the ATF when shifting, but it didn't get any worse either.
Service manual says the tranny takes 1.6 qts. But then the same manual also says the crankcase (with filter) takes 6.1 qts. so go figure.