For worn pumps and seals, the newer lightweight ATF's are no good. Keep with the older specs. If looking for synthetic, the redline D4 is definitely better off than the D6 and the hi-temp stuff is even more viscous yet.

If you're really trying to get the clutch packs to clamp down on a worn out trans, the Racing ATF is a synthetic version of Type-F fluid (high viscosity, no friction modifiers for less slip, high resistance to thinning) and what I'm using for a trans that would lose 4th then 3rd as it got hot while being driven aggressively; I later found other issues as well but increasing the viscosity helped immensely. With the superior shift kit springs, a modulator adjustment, and the special fluid, it shifts pretty muscle car like. It's pretty hilarious actually, 2nd chirps tire and the 3-4 will now snap off every time at WOT. The heavier fluid even made a difference in torque converter stall speed... huge believer in the heavy weight stuff.

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{2010 Mazda 3 266,000mi} - daily since new, Airlift suspension + some chunky sway-bars: fastest pass 17.7@77MPH
{1987 300SDL 256,000mi} - stock turbo @ 22lbs straight piped + a whole bunch of pre-turbo water/meth injection, 722.3 with increased line pressure, special sauce, and shift kit: fastest pass 16.5@82
{1992 964 Carrera 4 Frankenstein ??,???mi} - 993 forward dated, Speedster conversion, GT45 + E85: fastest pass 12.5@113