I'm not sure of European requirements for emissions, but if your car has a functioning DPF and SCR system and no mechanical failures the tips on your exhaust pipes should be incredibly clean no matter what you dump into the fuel tank. I worked for a company with 300 diesel powered (80,000 pound GVWR) trucks, the only time the exhaust tips got dirty after SCR and DPF was introduced was if a turbo failed and overloaded the DPF. We never put anything in the tanks other than #1 or #2 ULSD diesel fuel, depending on the weather. We even had a Volvo rep come check the VN370 tractors with a machine to test emissions - the results were the exhaust was not only cleaner than the air going into the air intake, it contained enough oxygen that you could breathe it.
Increased Cetane levels and cleaner injectors - always good though