Would like ability to track DEF purchase/use
 

I'd like to be able to record purchase of Diesel Exhaust Fluid and to graph DEF/Gal and/or DEF/mile

You could just add another vehicle and name it DEF. Other than that depending on what you are driving and how techy you want to get the ECM keeps track of that and can be read from the OBD with the right software or proprietary scan tool, although you won't get purchase history.

That would foul up the mpg figures for all those following that model and year stats.

You can just choose "OTHER" for make, model, engine etc etc

It's a Kenworth, there aren't (or I haven't found) a consumer available interface to the computer.
I don't want to have a second vehicle because that adds more fiddly nonsense to recording my fillups. I could also write it down in a notebook, but the whole point of this thing is to make it easier.
For now I'm using Note records, but it'd be nice to have it more 'integrated'