Pontiac: Transmission slippage, transmission slippage, 3800 v6

Question
I own a 1998 Olds intrigue with a 3800 v6 that has 126,000 miles. I recently changed my tranny fluid to mobile 1 synthetic. This is the third time I have changed it over the last 4 years. The last two times I used regular fluid and I had no problems. This time I used synthetic and The tranny is slipping now. It only slips when I am at a stop and I hit the gas kind of hard. I quit driving it after two days and I am going to  drain the tranny fluid out and put the regular fluid back in. Have I caused any damage? What should I do?

Answer
First of all you should never mix fluids. Unless you drained all 12 quarts of old tranny fluid out before you put the synthetic fluid in, you mixed them. I would suggest that you take it to a shop that can flush the trans and only use Dextron III in it from now on. You need to do this before you can tell if you have done any more damage. Also the filter should be changed because it is contaminated now.