Which transmission fluid should I use

I am a self maintainer of my automobiles since I got my first car. I am currently driving the car listed above it is the LS model with a 3.8L engine. I am going to change the transmission filter, and as much fluid as possible. The agent at NAPA (seemed like a professional-not overly anxious to sell) told me I needed Dextron VI Synthetic fluid, he even double checked this with a Chevrolet dealership up the street. I have a problem. My manual states to only use Dextron III. Which fluid should I use?