thorn,
There are a couple of different ways to handle the "keep the DEF fluid topped off" issue.
If your SCR (Selective Catalyst Reduction) system is working correctly, there is no need to constantly be checking
that the DEF fluid is full.
When the DEF fluid reaches the low threshold level you still have several hundred miles that you can go before the DEF fluid
runs out and you go into the "SCR countdown mode" where you only get 20 starts and then you can't drive the Sprinter and it needs
to go to a dealer and have the DEF refilled and the SCR system reset.
As long as you add at least 2 gallons of fresh DEF fluid between the time the "low DEF read your operators Manual" warning shows
up on your "Messages" (every time you restart your engine) the SCR system will reset itself, and you will be "good to go" until you
get the low DEF message again.
If the system is working as it was designed to do, there is absolutely no reason to be adding DEF to keep it full.
Besides which, if you are constantly refilling the DEF to the top of the fill tube you will be spilling it pretty frequently, which will eat the
paint off the non-plastic area that the DEF comes in contact with.
I just drive mine until I get the DEF low message, and then stop at a Tractor Supply Co. Store and by a 2.5 gallon kit of TerraCair Ultra
Pure DEF fluid and add it to the DEF fill.
In one to three engine start stop cycles, the SCR system will reset and I'm good for another 4k-6k miles.
Others seem to like to keep filling it up....but in my experience with 2010/2011/2012/2014 Sprinters (all of them having the Blue Efficiency SCR systems that
require DEF fluid) it really makes no difference as long as the system is working as designed.
Plus you can get DEF fluid at most Auto Parts stores and at WalMart...so you are never really far from a DEF source.
It's slightly cheaper @ WalMart, but Tractor Supply Co. has the TerraCair Ultra Pure DEF for about the same price as the Peak Blue DEF (in the
awful, hard to pour from, card board box with the leaky spout.
Hope this helps,
Roger