Part of what I felt was my responsibility to dad as his attorney not just for finance but also his health and welfare was to visit regularly to check on him..the NH...his needs etc. I clocked up 10,000 extra miles a year for each of the 3 years he was in his NH in doing this. Dad was self funding and in a reasonable financial situation so I felt it reasonable for his funds to pay for petrol. Had he not been I would have absorbed those costs myself but not everyone can do that.My car was only used for visiting dad so it was easy to keep seperate from my own personal use and petrol costs with a different car. I didn't though expect that he should pay for the inevitable additional wear and tear on the car or any mileage amount. However if you feel you can justify it and keep accurate records how likely is it that anyone will challenge your decision?