Why does gm want the transmission temperature to run hot?
EVERY carmaker prefers the motor oil, ATF, & coolant to live between 212F-221F.
H2O quickly boils off, heater performance is maximized, and motor oil & ATF have lower viscosity @ 212F-221F, which is worth like 2-3MpG.
2-3MpG may not be worth much to us, but to a carmaker it's 2-3MpG multiplied by however many vehicles they sell.
Before active aerodynamics, highway operating temps as low as 185F on the highway were TOLERATED by carmakers.
With active grille shutters and whatnot, highway operating temps can be raised which lowers motor oil & ATF viscosity for better highway MpG.
However, even with modern motor oil & ATF, keep in mind that the reported temp is NOT the hottest spot, it's the easiest spot to put a temp sensor.
Bet that if the motor oil or ATF temp is reported 212F, somewhere in the engine/transmission it's hotter, even if only for a moment.
So if the ATF is reported 230F, it's time to drop the pan. (Some motor oils start offgassing @ 239F - offgassing is basically oil consumption.)
