A work collegue put 1/4 of a tank of petrol into a Peugeot HDI diesel before realising.
He asked at the kiosk what to do and they said fill it right up with diesel and it will be O.K.
He almost made it to the end of the full tank before the engine expired..

If you keep topping it up with diesel you / he may get away with it, but I suspect the damage is already done if it's a modern high pressure (2500 to 3000 psi) injection pump.

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