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

That is driven only in summer , mostly street with 2 or 3 days at the track . Not interested in hardest shift , but longest life , cost of fluid is not a factor , as a rebuild is always 20 times the cost of the most expensive fluid , cheap insurance .
And I read the other threads, seems no consensus , but there were different needs in that thread , ie hard shift , best ET ,this is for long life street driving .
Are syn fluids recommended for a 727 ?

Thanks