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Diesel fuel pumps operate on very fine tolerances and at very high pressures – modern systems run at between 350 and 1600 bar) – and are lubricated by the fuel. Petrol in diesel acts as a solvent, reducing lubrication, and can cause damage to the pump through metal to metal contact.

Metal particles from the damaged pump can be deposited in the fuel causing further damage to the rest of the fuel system.

Some fuel system seals can be affected by the compounds in petrol too.

The further the contaminated fuel goes in the system the more expensive the repair. In some cases it can be cheaper to fit a new engine!

Common rail (or HDi) diesel engines are particularly vulnerable – if fuel contaminated by pump wear debris gets as far as the common rail system you may have to replace the low and high-pressure fuel pumps, injectors, fuel rail, line filters and the fuel tank.

Gasoline Direct Injection (GDI) petrol engines are particularly susceptible to damage too.

Unit injectors (VAG PD engines) are not as susceptible since they are oil lubricated and no high pressure fuel pump is required.

It used to be common practice to add petrol to diesel in colder climes to stop the fuel from gelling in winter.

tbh, lots of diesel cars suffer a whoops! moment when a few litres of unleaded is inadvertantly added. Most survive for many more years. As long as the petrol is well diluted it is rarely catastrophic.