First, read this thread which is later and more detailed than the other one.
Either Dexron III or T-IV will work. The Valvoline is essentially T-IV and it is partially synthetic, and it is according to manufacturer's specs as being the correct, recommended fluid.
When you drain any automatic transmission you are only draining out about 1/3 of the entire fluid capacity of it. It is impossible to drain it all out without having special equipment. However, you grossly overfilled it. But you can simply drain out what you put in and recapture all that fluid. Use a clean pan to drain it into and save it all. If it is clean (which it probably is if you didn't drive it much) simply retain it and (this time) pour in only about a pint at a time, checking the fluid level on the dipstick between pourings. It will take about two quarts at most. Save the rest of the relatively clean fluid for the next time you change it.