Diana, get the car scanned for stored codes. MY Check Engine light never came on yet once I scanned the Park Ave I had P1811 and other codes in there so post back the codes after getting car read with a good scanner.
TransTune will clean up the valve body and remove remove varnish-recommended.
I drove normally with the TransTune in for a couple of weeks before installing the shift kit.
Every thing was clean as a whistle in there.
You should be able to get Dexron VI form any good auto parts store or GM.
I also use only Delco or GM tran filters as well.
When I did the shift kit I started by adding 5 liters then driving and checking levels,I used just under 7 liters and I am on the full line.
I also always use a bottle of synthetic Lube Gard "red" in there as well.
I generally buy 6 bottles to have on hand
Link LubeGard Red: LUBEGARD Automatic Transmission Fluid Protectant