16 Dart SE with the 2.0 engine. I've got 90k miles on it. I drive it 50 miles a day. I'd say 70% interstate 30% in town.

The owners manual says to change fluid at 50k and 120k if you drive in extreme conditions.(then they list those conditions) Otherwise they say nothing about a service under non extreme conditions. I know the filter can't be replaced on these transmissions without pulling the trans and disassembling it to some extent so a service would just be a fluid change. Under normal conditions, should the fluid be changed or should it be left sealed?

From what I've found online is that when people are changing the fluid, they are just measuring exactly what they remove and putting that amount back in.

Also, the plugs are original. They are supposed to last 100k but I'd like to pull them out and have a look at them now.(probably should have done it sooner) What is a good brand to replace them with? Get them from Mopar or is a parts store replacement plug OK? Advance Auto lists Autolite/NKG/Denso and Champion. All say they are an Iridium plug. Thanks

Generally, if the manufacturer doesn't life a change interval then it's designed to last the life of the car. However, what they consider the life of the car and what you consider it to be is probably different. So you can't go wrong if you follow the severe service recommendation, even if your driving style may not be considered severe.

As for the plugs, the factory one lasted this long without an issue, probably not a bad idea to go with those. All I now is that my 413 did NOT like Bosch platinums back in the 80's. The rest of my rides run copper plugs.

Well, I've heard of people having problems with their transmissions shortly after having a service done. Something about the new fluid washing away some of the linings on higher mileage units. They would have been better off just keeping the old fluid in there. I didn't know if 90k is in that range of "just leave it alone" or if it should be OK to replace.

ZF 8 and 9 speeds use the same fluid. ZF (depending on where you read it, it varies) recommends service between 40 and 60 thousand miles, or 100,000 km. Fill for life (Chrysler's term) means to me when the trans fails, that's life, c'mon in for a new car. So we're spending the bucks on 50,000 mile service intervals. My local dealer only does flushes, my transmission guy says, no way, so we change the pan (integral filter) and fluid, no flushes for us.

The German top manufacturer of transmission ZF recommends oil change every 8 years or 100,000 km. The oil change interval is actually set by the type of the transmission and also by the strength, or how long it is designed to be.

The info in the link is new to me, you might find it helpful?

https://gearsmagazine.com/magazine/the-zf-8-speed-transmission-fluid-and-filter-change/

I have a 2014 Dart 2.4L that hit 100K miles a few months ago. Mine's a stick so can't help you on the transmission.
I changed plugs shortly after I hit the 100K mark. My car seemed to run fine but when I pulled the plugs the electrodes were worn down to the point several had almost none left. Based on that, I would say that maybe 80K is a more appropriate interval to change them. I used NGK. So far so good. I don't recall what OEM were on the Dart, but NGK are factory on my 5.7 hemi Challenger which I'd changed a year or so back, so NGK was the brand in my mind as being the way to go.

I'll pull a plug and see what brand they are. Thanks