here's the link to varnish build up at 220 F idea in the forum thread
https://www.gm-trucks.com/forums/topic/225822-transmission-temps-how-hot-is-too-hot/
here's the generic trans fluid temp chart attached
https://www.gm-trucks.com/forums/topic/225822-transmission-temps-how-hot-is-too-hot/
here's the generic trans fluid temp chart attached