I said "I don't know how fuel quality testing is done at the fuel pump in Germany."
and you replied
They do check in Germany, especially the tax authorities, but nobody wants to even think about getting in trouble with them.
Sounds like this deters the criminals, but perhaps not enough deterrence for insufficient care in cleaning tankers. If your dealer knew about other cases 4 years ago, perhaps they were local, and came from the same gas station you used, or were filled from the same tanker.
Anyway, it seems like you have done all you can.
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View my fuel log 2014 Mirage ES 1.2 manual: 46.0 mpg (US) ... 19.5 km/L ... 5.1 L/100 km ... 55.2 mpg (Imp)