My Alfa 159 has the advanced 1750 Turbo which is also used in the Alfa 4C. It runs on 95 Octane as recommended. The engine is not designed for 98 Octane and would be worse. The Octane rating is about preventing early detonation in high compression ratio engines. You need the higher Octane to cope.
If an engine is designed around 98 Octane then the use of 95 Octane results in poorer performance as the knock sensors retard the timing to prevent detonation. It's probable that the V40 falls into this camp.
The quality of the fuel may also be aligned with the Octane rating but there is no reason for this. The same additives could be put in the lower octane fuel.
So with or without additives if the engine is tuned for 98 Octane then you will get better fuel economy and performance by using it.
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