My own admittedly uninformed understanding is that there was cheating all around, but VW was by far the most flagrant. BMW and Mercedes diesels required DEF (di

My own admittedly uninformed understanding is that there was cheating all around, but VW was by far the most flagrant. BMW and Mercedes diesels required DEF (diesel exhaust fluid), whereas VW did not. Marketing and/or executive leadership (again, as per my understanding) pushed the notion that the additive would make diesels appear to be less attractive and who wants to add a second liquid beyond fuel every few hundred miles? But weren't they all cheating? The diesels with DEF were still above the legal limits, albeit to a lesser degree than the VW's who didn't even bother with DEF because, I guess, if you're going to cheat anyway... How incorrect is my understanding?