Thanks Rock.
In the SCR cars, when the DEF is low, the information display will tell you ADBlue low. It will eventually tell you 20 starts remaining and count down to zero starts. The fluid is available almost anywhere diesel fuel or auto parts and accessories are sold, it is fairly simple to fill. However, Mercedes has its special filler neck to interface with its own ADBlue bottles to tell the computer you filled it. So, you buy one 2liter bottle from Mercedes, pour that in, then use the top of that bottle for a filler / funnel to fill with VW or generic, your choice.
So far the dealer filled mine at 10,000 miles, only needed 4gallons to fill the 10gallon tank, I'm not expecting it to be a frequent service item if the tankful lasts 25,000 miles.

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Gone to the dark side

- Jeff