It won't do any harm. I poured a bottle of the Platinum in my Ranger when I did a pan drop/filter change with Mercon V. Obviously a different trans, but one of the same era in transmission design and manufacturing. I haven't noticed any difference in how the truck shifts (a good thing), and temps may be slightly lower, though it is hard to tell because this trans has never been over 190*F anyway according to the Scangauge.
A coworker used the "Instant Shudder Fixx" version (think concentrated red) in his Expedition 4R70W that had some shudder at 250K+ miles, and it almost completely eliminated the shudder. He took the Expedition on a 2K mile trip recently after adding the Instant Shudder Fixx and it did well.
I am skeptical of transmission fluid additives as transmissions tend to be a sensitive component, but Lubegard seems to make quality products that work as intended. I have never heard a complaint about their products.