Here's what happens to ATF with temp...
View attachment 1715055847
This is why a hot ATF temp reduces stall speed. With a much lower viscosity, the fluid flows faster and more easily causing the converter to more fully transfer the power to the output shaft at a lower rpm. Add in more engine torque and the fluid resistance drops too. That's why the same converter in a high torque engine has a lower stall speed than it does in a lower torque engine.