I'm on a long road trip at high altitude and thought I should get some def for my x535d just in case. Can't find it anywhere online. Does anyone know if the rumors I've seen that Audi, VW, MB and BMW all use the same bottle and chemical are true? Can I just buy some at the local VW dealer where I am? There's no bmw dealer nearby. Or can I buy a two and a half gallon bottle at a truck stop and refill with that in a pinch? I don't have a warning light now, just trying to be prepared for when i'm 1000 miles from nowhere and it runs low.