One of the bizarre outcomes of the "mandatory' catalytic converters is the change in the smell of diesel exhausts.
On the old days, diesels just smelt like, well, diesels. with about the only variation being the "colour" of the smoke when starting or if you floored it.
After catalytic converters, there was a distinct sulpherous smell, which in Australia, should be odd.
Australian crude has the lowest sulphur content in the world.
Stuff from the middle-east among the highest.
If your exhaust smells like rotten eggs or worse, it's the fuel, and your catalytic converter.
Older diesels will run on just about any "light" oil, even filtered oil from the fish and chip shop. Do not try this with your new Mercedes. "Bio-diesel" is a way around shortages / the spiraling cost of proper diesel fuel, so I can imagine the Eurocrats going nuts over the thought someone, somewhere, might be "minimizing" their contribution to the government coffers. As H. L. Mencken defined it: "“Puritanism: The haunting fear that someone, somewhere, may be happy.”