Top Options:
Valvoline Import Multi-Vehicle Full Synthetic ATF: 100C KV of 6.9 cst. Valvoline recommends this for vast majority of import cars, as well as GM D2, D2D and D3 applications. With its higher viscosity this strikes me as a full synthetic D3 fluid with modern additives. (datasheet metalink attached)
Valvoline Extended Protection Full Synthetic ATF: Most premium offering of Valvoline, that is suggest bot for D3 and D6. Its 100c KM is 5.9 cSt so it is on the lowest end, but it might be that the its base oil quality ensures good film properties that protects while lower viscosity means less temperature. But this is just my speculation, so this would be my second choice here.
If the transmission is leakign or already has shifting problems:
Castrol Transmax High Milage: Synthetic BLEND, high viscosity and seal swellers, so seems to be a good option for leaking transmissions.
Some options that I have looked into but decided not to be a good option:
Castrol Transmax Full Synthetic Multi-Vehicle ATF: Low viscosity. Might be a good choice for a daily driven modern/new vehicle for efficiency, but definitely not for a HD / heavy vehicle.
Castrol Transmax ATF/CVT Universal: Kinematic 100c viscosity of 7.1 cst, Full synthetic, D6. But rated for both, so its frictional properties would not be optimal. Compared to CT D6 Mercon LV, this should be a better fit as Viscosity lines up better, but frictional property does not therefore an inferior choice for clutch health.
Castrol Transmax DEXRON®-VI MERCON® LV: This is an LV (5.9 cst), so again, for modern vehicles. Compared to Castrol Transmax ATF/CVT Universal, this should be a better fit on frictional properties, and an inferior choice on the viscosity side.
Valvoline DEX/MERC (ATF): D3 fluid, not synthetic.
Valvoline DEXRON VI/MERCON LV (ATF): This seems to be very similar to Castrol Transmax D6 MLV. Viscosity of 6.0 cst and D6 Approved. So for people who really value the official approvals, this might be a good choice.
Valvoline MaxLife Multi-Vehicle (ATF): This is essentially a low viscosity (5.9 cst) fluid with seal conditioners that is dual rated (cvt and stepped). If leaks are present, Castrol THM seems to be a better option.
Transmax Import Multi-Vehicle: Good kinematic viscosity (7.4 cst), but I don`t think this one is full synthetic (it says synthetic technology). Seems to be targeting Japaenese cars mostly, but Castrol suggests it for D3H as well.