I'm in the "Oil lasts as long as the life of the transmission" camp and quality synthetic oil lasts around 100k miles, unless you get it hot then less. Anything after that is "borrowed time" which is why you are now feeling slipping at 160k.

Changing the fluid won't make it better, but it may have fail less quickly. I highly doubt a 2015 transmission is going to fail sooner because you swapped out the fluid.

Here is the diagram of the 4HP22 I'm thinking of rebuilding in my 1999 Discovery. I see nothing here that can be "held together by fluid" or "dislodge and fail". Maybe the orings can fail with a swap, I'm not sure.

I did do a drain and fill on it at 90k and it failed at 150k. Naysayers can say my fluid swap caused the failure, but the failure I had wasn't a fluid swap issue, it's a design flaw in the transmission and I was never left stranded. It still shifts fine, just not automatically.






I'd do a drain and fill on your truck. If you are lazy, that's good enough. Someone ran samples on his BMW 540i, and one drain and fill was 70-80% new fluid. The second drain and fill was 98% new fluid. The third got him up to 98 or 99% with only a small amount of old contaminated oil left. I can't remember the actual numbers, but it made zero sense to drain and fill the 3rd time. Two times was really, and since yours is so old, I'd recommend that. But if you change every 100k, once is probably enough.

Most people say driving 2k miles or so is best before the 2nd drain. That's probably excessive, but the point is to drive it and mix it up before the 2nd drain and fill.

I did a replacement on my 2500HD after 65k miles (I tow with it), most of my other rigs I do a drain and fill at 100k.

I used to do transmission filters and pull the pan, but there is hardly any crap in it at 100k miles on a modern, well maintained rig that I think you are doing more harm than good pulling the pan. I might do it at 200k, I haven't decided yet.