You will NOT void your warranty if you run biodiesel. BUT dont expect them to honor a fuel related repair if you have over B5 biodiesel in the tank, most common biodiesel problems with late model diesels is that they dont take very well to bad fuel, whether it be diesel or biodiesel.
Biodiesel on the other hand doesnt have mandated standards, so it can vary in quality. To be 100% sure ask the vendor for the test sheet from their recent batch of biodiesel to see if it meets the ASTM standards. If they cant provide you one then run the fuel at your own risk.
Can they test your tank for the amount of biodiesel in your tank? YES! but its an expensive test, dealers wont waste their money on a $500 test at every oil change just to screw with you. But if you happen to come in with a bad injection pump and they suspect biodiesel was the culprit they will have the test done to see if your over the B5 limit.
But you can always siphon out the bad fuel fill it up with diesel and they will have to honor your repair
