DEF (Diesel Exhaust Fluid) is DEF, right?

[Best4x4xFAR]

First, this has probably been covered, and if so I apologize, but my search skills must be letting me down. I found a lot of discussions on fluid usage, but not much on this specific question..

So the first time the DEF 'No Start' message came up, I purchased BMW DEF Fluid for the refill. But is it really any different than Peak or Shell or any number of other brands of DEF fluid?

Manual of course 'recommends' BMW DEF fluid, saying, "Preferred: BMW Diesel Exhaust Fluid." but then goes on to say, "Alternative: NOx reduction agent AUS 32"

BMW of course can't require you to use their fluid, unless they are willing to supply it free of charge..

So is there really any advantage to paying roughly twice as much for the BMW branded fluid over a fluid that carries the 'AUS32' rating?

I don't mind purchasing the BMW fluid online from FCP Euro or other, but the Peak and Shell DEF fluids are readily available at almost every store, and are a bit less than half the price. If its really all the same stuff, why bother?

That brings me to the other question, shelf life. Seems like I hear the 'one year' life mentioned a lot, but is that for a sealed unused container, or only apply after it is opened?

Appears that the DEF fluids available in local stores have manufacture dates, but not expiration dates. Shell states a two year shelf life on average for this product, but notes it is dependent on storage location/temperatures..

All of this makes me think its best to let the system run down to the no start warning before refilling, in order to keep fresh fluid in the system, instead of topping off opportunistically, to avoid getting the no start message.

Furthermore, BMW dealers get this stuff in bulk. And I don't think they have a lot of Diesel vehicles coming through the department. So should you be worried about the dealer's supply going stale if you're getting the 'free top-off' with service?

Okay that was more than I intended, sorry..