I have been using 5w-30 mostly since I got my 2018 6.2L new. My logic was this engine is very similar to LT1, and 5w-30 is recommended there, and protects from wear better. Any gas mileage lost was worth it to me since I knew I was going to keep it well past warranty period. Oil analysis over the 103,000 miles has shown it works fine. 0w-20 also worked when the dealer performed oil change a few times, so long as I kept oil change interval to less than 4-5k. Not enough samples of each at consistent intervals to be scientific, but good enough for me. Not sure I’d go to 0-40, but good to know 40 an option for super hot conditions.
add: anyone switched their 8speed from the LV (low viscosity) ATF to the normal ATF? I assume it was spec’ed for similar reasons, with similar wear risks. I use the Amsoil LV ATF now, and it seems to work well, but maybe the high viscosity stuff can get me to 200k?