I have never put an additive in any trans I've ever owned either stick or auto. Stick should get about 3 qts of Mercon V fluid and call it a day. Pull your shifter and pour the fluid in through there to make it super easy.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.