What is the best transmission fluid to use in my M6?

I have heard it is best just to use the Dexron 3 (or 4), and stay away from the synthetics for the T56's. Anyone else heard of this? What is the best transmission fluid?

I've been using RedLine Synthetic for years. No problems. But I don't know
if its "the best".

I put castrol dex 3 in mine

Ive had Valvoline Durablend in mine for 30,000 miles. No problems

i have mobil 1 syn. no problems here.