I have never needed a mfg warrentee, as far as I'm concerned, they will always put it on painter error, so why bother.
My combos are by feel, and I get a bit of the characteristics of what i blend ( hey like they do with wine!)
Not suggesting this to anyone else, just I have found ways to make it work. Not for the timid.
As I remember right, you thought that only BM extender should go in Aura. I can tell you from experience that XIM is just as good at extending/ drag reducing, if not better. I add just about the same H2O as extender, and the stipple closes up so better coverage.
Its all through experimentation to see what works for me.