Read the owners manual, and go with what it says, if you drive like a little old lady only on sunny days and never off-road, bad weather or dusty condtions, then you can use the "Standard Duty" cycle, which I think is 120k miles, the "Severe Duty" I think is 60k miles, and I would change it when it says.
The manufacturer considers 100k mi the life of your vehicle, so keep that in mind when they say its a lifetime fill, it may mean it will last to 100k mi and if you want to keep the vehicle longer than that, you'll have to change the fluid.
Do NOT flush, just change the fluid and filter in the pan.
The Dipstick for the NAG1, mercedes 722.6, is a special tool NOT a dipstick, it is NOT designed to stay in the trans while you drive. It will stick out another 2 feet from the tube and will NOT seal. As well, its designed to be bottomed out on the pan to measure fluid level, unlike dipsticks that bottom out on the cap on the tube.
You'll have to improvise something if you want to leave the special tool in the fill tube as a dipstick, and then you'll have to be ultra precise, because it will have to bottom out on the bottom of the pan as the cap seals on the top of the tube. And this is for a trans that has to be ultra precise on fluid level.
BTW, you'll also have to come up with a way to measure the trans temp, the fluid level is based off a precise level in millimeters based off the trans fluid temp, you look it up on a chart in the FSM, but you can find it on the internet with a google search.
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