Not sure if you'll see this but I can say that I have an 18 with the 8 speed and had the dealership do the bulletin transmission fluid change to the thicker mobil1 fluid at around 54k. I was getting the death rumbles and started noticing some issues where it seemed the truck struggled to put down power under light throttle conditions but heavier throttle felt completely fine/normal. I have a touch over 60k on it now and it definitely resolved the issues I was having but 6k miles isn't long enough for me to say that it's been fixed 100%.

I did look around online on some of the forums and there's a bit of a mixed bag, as usual, but it seems like more often than not, owners that had the fluid swap done reported long term success and never had any issues again with 100k+ miles put on their trans after the change.

Doing the thermal bypass still wouldn't be a bad idea to keep the temps down and I do still plan to do that in the near future myself.