Syn ATF is not overkill, not if you want the transmission to last a while longer. Dex-VI also works well, and is rumored to be part-syn.
What I'd do right now is drain the pan, re-fill with some cheap ATF, run it a week or two, change it again, add Auto-RX and follow the directions, then drop the pan, change the filter, and flush with the full synthetic ATF. This worked wonders for my Buick LeSabre's transmission.
My reasoning is to get some of the cruddy old fluid out, clean the transmission up a bit, then hit it with all-new fluid. Kinda like changing the water in a fishtank. You don't want to change dirty water all at once, otherwise you'll kill the fish with the clean water they're not used to.