If you have a stock TC there is now way to completely get all the ATF OUT , you can remove up to 12 quarts of oil. After you remove the pan loosen off the 10 valve body bolts (7/16" heads)approx 4 turns each. This will allow you to drain 1/2 the oil that is in the TC. Most synthetic oils are mixable with regular ATF , most after market TC's come with a drain plug so you can change most of the oil.