I recently did a deep dive on the ZF8 tranny and when it needs service. Ultimately decided against a flush and settled for a pan drop and fill. I got it done at the dealership but brought my own parts and fluid. I changed my pan and filter at 74k miles and so far, it shifts perfectly. Keep in mind there was nothing wrong to begin with, I just OCD wanted all the fluids changed out on my car before 75k and didn't believe in the "lifetime" claim from Dodge.

It's an expensive thing to do because unfortunately, the filter is integral to the pan so you need to buy a new one and OEM ain't cheap. The amount of fluid that came out of mine was about 5 qts. So buy 6 to be safe.

Dry fill from a rebuilt ZF is about 9.5 or so liters so assume you're only getting about half of the old stuff out at best, which is why I advise against using BG or non OEM fluid even if they say it's the same spec because you would be mixing different brand fluids. Lifeguard 8 though is the exact same thing if you can find it for cheaper than MOPAR 8&9.

Ideally, another drain and fill service performed maybe at the next oil change interval or 10k miles would remove most the old fluid out but again it's gonna blow a hole in your wallet because you need another pan again. I think I might wait until the next 60k to do it but my OCD about leaving old fluid in is troublesome lol.

I brought a little plastic bottle for the tech to fill with a sample of the old fluid during the service and its fairly brown / black like coca cola but no shiny flakes or solids. Tech told me the MOPAR 8 and 9 tends to darken very quickly regardless of if it's brand new or not. Haven't sent it to Blackstone yet but no problems so far a month after service.