Original Showa SS8 (SAE 10W) fork fluid has a viscosity of 36.47 cSt @ 40°C (104°F), 7.24 cSt @ 100°C (212°F) and 167 viscosity index. It also looks and smells just like ATF.
Fasteddiecoperman used WalMart's Super Tech High Mileage ATF made by Warren Performance Products (WPP), most likely the equivalent of http://www.wd-wpp.com/greases/greases008.pdf
which has a viscosity of 34.5 cSt @ 40°C, 7.3 cSt @ 100°C, and 187 VI... Viscosity and viscosity index vary between different types and brands of ATF, and between different brands of 10W fork fluid.
Fork operating temperature is normally at about 26~33°C (78~91°F) in 21-28°C (70-83°F) ambient temperatures.
With maximum or near maximum fork spring preload, I get sufficient rebound damping and motion control with 12.5W, a half and half mixture of 10W and 15W fork fluid.