What is the best trans fluid these days for an A-727

I'll ask my transmission guy , he's been in business for 35 years , but can't imagiine how syn fluid would be hard on seals , they said the same about syn motor oil back in the day , and i have never had a seal leak on very high mileage cars . My last 4 cars had over 450,00 km with no leaks . Just not sure about such an old cars though .
Same old trouble who to believe , engineers that design and spec things or the guys that repair build , always hate these arguments .