Is super unleaded gasoline better for a car with over 250000 miles?

Answer 1

The mileage on the engine has nothing to do with what octane fuel it may require unless the engine has heavy carbon deposits which can raise the compression ratio which may require the use of a higher octane fuel to prevent pinging or pre-detonation. Normally use exactly the octant fuel as recommended by the manufacture. The use of a higher octane fuel is a total waste of money and will provide no benefit whatsoever.