Charge the company car driver for the repairs they will never do it more than once. The company I used to work for had the following policy on misfuelling.
The employees cost centre would be charged the cost of repairs and this would be recovered from the employee by monthly payment over 12 months.
If you put diesel in a petrol or vice versa then your IQ must be lower than that of a goldfish IMHO
Our firm doesn't take such a hard line - our legal counsel are more cautious as, unless the employee gave their written consent, it could be seen as an unlawful deduction from wages.
And, of course its inevitable that it happened to a handful of senior people.
It's not intelligence but concentration, and external distractions that can make a split second wrong decision - kind of like being involved in an RTA, and our firm would not normally recover accident damage from employees.
One or two of our culprits have been international assignees who may not be as familiar with the UK definitions of fuel types when they pick up the keys from Terminal 1...