My son in-law put 1/4 of a tank of unleaded petrol in his Fiat diesel van before realising he had made an error and then filled up with the correct diesel fuel.

He has driven the van since and has two conflicting pieces of advice on what to do now: one garage says he must stop driving the van and have the tank and system drained - at a cost of £150 - and the other, an independent mechanic, says the ration of diesel to petrol makes it less critical and he can continue driving but not to over rev the engine but to add upper cylinder lubricant regularly. What do you suggest he