http://www.autoblog.com/2008/03/13/m...uvs-this-fall/

sample quote:

"A small AdBlue tank is sufficientThe urea solution is held in a separate onboard tank, and fed to the metering module by means of a pump. As the average consumption of AdBlue is only around 0.1 litres per 100 kilometres (or 1 to 3 percent of diesel consumption), the tank can be of such a size that it only needs to be refilled at the workshop during regular service intervals. The tank capacity is 28 litres in the R 320 BlueTEC and ML 320 BlueTEC, and 32 litres in the GL 320 BlueTEC. Even under more demanding conditions with a higher AdBlue consumption, e.g. when towing a trailer, this is sufficient to avoid the need for unscheduled refilling stops in the workshop. This means that the customer has no need to concern him- or herself with the AdBlue level in the tank. Should the AdBlue level unexpectedly fall below a certain level nonetheless, the electronics provide a timely warning."

Yep, I know what you are already thinking,
and unfortunately
you would have to boil your **** down to about one third
to make its concentration strong enough

gives a literally true meaning to the old saying
'**** poor' engine