I've had it happen to me twice. On the first occasion, I hadn't pulled off yet when the mistake was discovered. The garage paid for a mechanic to drain the tank on the spot, and while I waited, I was given a free meal at the Wimpy. on the second occasion, my wife drove a kilometer or two. The garage paid for towing fees, a mechanical inspection (fortunately there wasn't any damage, other than the system needed to be flushed) and they paid for it all. Although there are signs stating that the owner of the car is responsible, it is the driver's word against the attendants' word, and it is always the driver's word that is taken above that of the attendant. Engen actually dismisses attendants who make that mistake. If a garage disputes it with you, call the Petroleum company concerned - they'll sort it out for you. But you should take care anyway - it is downright inconvenient when it happens, and you may have to pay for everything yourself until the garage's insurance pays you out. In my second instance, it took only two days for them to pay, luckily.
36 mths ago