Originally Posted by pizzaguy

You could make a mistake like that, but I would not.

IMO, what the car needs is what we call a "Transmission service" where they replace fluid and filter. Your car is 19-20 years old, it's way past time, this should be done every 100,000km at least. Most let it go until the transmission acts up, which seems like neglect to me. My car got all new fluids (ALL) in2016 right after I bought it, at around 31000 miles (45,000 km or so). In fact, it's pretty much time to do it all again.

I would NEVER put additives in an automatic, just new filter and new fluid.

As to taking it to a garage or not, that depends on your comfort level with the procedures involved in auto trans service. The Manual is super easy, auto not so easy.