Dex VI is superior, as long as it works well initially. This just came up yesterday, my brother is getting his dirt track car back together, being a Turbo 350 it's had regular Dex it's entire (abused) life, so he's putting Dex III/Mercon in it. Not sure if a full synthetic ATF would start leaking, and his external ATF cooler is actually an old A/C condenser, so temperature normally is a problem.