So I'm getting mixed reviews and answers when I search.... What kind of trans fluid for a 98 GS300?
And also some say that its not a good idea to do a trans flush on a car with over 200k miles on it. Is this true?

Thanks.

Any fluid that meets Mobil 3309 spec will work. But I happened to get a whole case of Toyota T-IV from the dealer for butt cheap so I just went that route.

A pressurized flush on a transmission that old can stir up deposits that will cause issues in the valve body and stuff. But you can still exchange all 14 quarts by pumping out old fluid from the transmission cooler lines and pouring new stuff in, one quart at a time. That's how I did mine and it worked great.

And is there a filter in this tranny?

There is a screen. I replaced mine but likely won't do it again.