The modern versions of this transmission (5R55E, 5R44E) use Mercon V, but I can't say for sure if that's what will work best in your transmission. Mercon V wasn't around until about a decade after your transmission was built.
I would guess that Mercon V would be okay to use in this transmission, but you may want to play it safe and just use a Dex/Merc fluid. Something like MaxLife Dex/Merc would probably be good.