98 Deville transmission fluid change

What does GM recommend on changing the transmission fluid on a 98 Deville, how many miles? I would guess, like most cars at about 25,000 miles, but don't know for sure until I get ahold of an owner's manual. Also do they use regular Dexon III or synthetic fluid coming out of the factory? Many Thanks!

They claim you can wait for the "service transmission" message to come on the DIC which supposedly occurs around 100K. I drained and changed the screen at 40K. Once a year I pump 3 quarts out of the filler tube with a Mighty Vac hand pump then replace with 3 quarts of Dexron 3. It might be overkill but trans fluid is a lot cheaper than a rebuild.