i've been told 190 degrees give or take is most efficient and ideal for your trans. That's why it is always recommended to plumb it through your rad too.
Like engine oil, 160 is too cool. But i'm not a transmission scientist.
i've heard more or less the same thing too, and like any oil that gets hot / cold / hot, you want to boil out the moisture, but here in so cal, i've yet to see someone have a trans die because it never got hot enough.
people buying a cooler need to consider where they are too
