i use synthetic fluid atf+4.in both my cars.9sec duster and 11 sec street strip duster.i think it depends on how much you race it.i run 2 classes,14-15 runs in a day.later rounds back to back.i foot brake and it seems my launch rpm stays does not vary as much when the trans is hot with synthetic fluid .
i have over $1000 in my transmission $4.00 a quart is not that big of a deal.