Dexron VI is an upgrade to Dexron III that came with the car new. Dexron VI is backward compatible meaning it can be used in all previous Automatic Transmissions (except manual trans, transfer case and PS).
Note - You do not need to use GM branded fluid, everyone has a Dexron VI ATF on the market. Use a name brand product and avoid the store brands which are made for a profit at a low price, quality be dammed. Dexron VI is a lower viscosity ATF with improved additives.
With a SC'd engine you need an oil that can with stand high temperatures so a synthetic oil is a better choice. Again, use name brands.