Synthetic ATF: Is there downside, what is upside?

[shertex]

Car has 178,000 miles, has always had regular ATF. Considering switching to synthetic. I don't care about smoother shifting (though I might like it if it was a result); main goal would be extending life of transmission.

Would this likely be the case?

And, as long as torque converter is drained too, is there any downside? My last tech strongly advised against switching, saying it could create problems. I have no desire to invite unnecessary problems, but I want to take the best possible care of the original transmission.

(I know there are some scattered posts on this...I've read them...but would like some more extensive input.)

Thanks!