Second the vote for synthetic ATF, with a caveat.

If you are going to do a complete flush, then go ahead with the synthetic.

If you are going to do a series of partial changes (just dumping what is in the pan), then get a case of non-synthetic and use it to get mostly clean fluid into the system. Then change to the synthetic.

This would be a great time to switch over to a fully synthetic fluid. Mobil 1 synthetic ATF, for example

The torque converter can also be flushed by disconnecting the rearmost transmission cooler line (the return line).

hook a clear plastic hose from it to a catch basin. Put the transmission into Neutral, start the engine. Let it run until your see air bubbles in the plastic hose.

turn off the engine, add 2 quarts of fresh fluid. repeat 3-4 times until all the fluid exiting the hose is fresh fluid.

reconnect the line, and top off the fluid level.

this method flushes out all but maybe 1-2 quarts of the 7 quart capacity.

i happened to use a Quaker State 4 X 4 Fully Synthetic fluid (found it at Wal-mart). Have clean, crisp shifts in my bricks and in my '84 Olds Custom Cruiser Wagon for more than a year now.

Dex III is the type(everything but fords and hondas use it. Brand doesn"t matter. You need to clean the screen inside the pan, and realize that you won't get most of the fluid out because it sits in the torque converter. You need to power flush it. Shops charge about $80 or you can buy a special hose from IPD and do it yourself.

Good luck