Why is it getting so hot? (Jetta JR) Pöl?

Hello everyone,
after I replaced the faulty coolant level sensor in the expansion tank (8 Euro VW original), I noticed that the temperature gauge is no longer in the middle, but at three-quarters.
I just replaced the thermostat and flushed the cooling system. Everything looked normal and there were no blockages.

Question:
Could it be that this is due to the fuel being contaminated?
I have been adding 15 liters of diesel fuel (Aldi 0.75€ per liter) to my diesel tank with every fill-up for a while now. The car runs much more smoothly than before, I'm almost always looking forward to the next time I need to refill it, because afterwards it performs much better.
However, I haven't been able to find any connection between Pöl and rising temperatures online so far.

Can anyone say anything about this?

Greetings
Hendrik

[Translating...]

Bist du dir sicher, dass das Golf 2 (=Jetta) Schaetz-Eisen auch richtig anzeigt ?

Es gibt ja 2 Fuehler am Wasserflansch. Tausch da einfach mal die Anschluesse, wenn er dann "richtiger" anzeigt, ist der Fuehler hin.

Wenn er dann immernoch zuviel anzeigt, sind entweder beide Fuehler hin oder die Kiste wird wirklich zu heiss.

Warum kann ich aer jetzt auch nicht erraten - sorry
m;