Scotty, I have a 2018 Chevy Colorado with the 2.8L Duramax. I do my own maintenance but I was dumb and believed the "never have to change the tranny fluid" so I haven't yet but I drive a lot and have 105,000 miles plus. I went into the dealer and they told me " oh you should have had that changed but now we don't want to do it because it might screw it up" is this true or should I change it now and then stay with it regularly? I do a fair amount of towing but mostly my miles are from the highway.
This is a high risk proposition. You have gone through at least three maintenance cycles with the trans without providing maintenance. If you decide to change out the fluid now, keep the old fluid so it can be reused if the new fluid messes up your trans. By comparison, what kind of shape do you think your engine would be in if you didn't change the oil and filter for about three maintenance cycles - 15-20,000 miles?
I hear you. There isn’t even a dipstick for the trans fluid, it’s so stupid. So I change it out, save the fluid and what signs would I be looking for that would mean I should put the old fluid back in, tranny slipping gears?
Change it now. Always save the old fluid incase a problem does come up.
Change it out for new and drive it. If there are problems and worse slippage, put the old stuff back in and top it up to full with new.