Transmission fluid is killed by heat. The normal temp is about 175F. For every 10F over that, the life of the fluid is cut in half.

It depends TOTALLY on your driving conditions. If you mostly cruise on the highway with very little stop and go driving then you MIGHT be okay with 45k mile changes using synth. If on the other hand you drive a good bit of stop and go traffic in ambient of 80F or better then you should be changing it MUCH more often.

How much did that new transmission cost? How much does ATF cost? If it were mine, I would be changing fluid only, in both transmission AND torque converter, at about half the normal interval and then fluid AND filter at the regular interval. If I were driving stop and go, I would be changing it so often that I would worry about wearing out the threads on the drain plug.

I am NOT a fan of automatic transmissions. I am a manual transmission maniac, but I do know how to make them live and have done so for other members of the family. The key is adequate fluid changes. These cars are VERY easy to change fluid and filter compared to many other cars. The difference is that they have drain plugs on both pan and torque converter. This makes it VERY easy and quick to do.

With an a/t the change it hot part does not apply, but the often part DEFINITELY does.

Good luck,