The cost of the frequent oil changes more than covers the premium fuel's
extra cost for those miles. It will be another 15 to 20 years at 7.5K
miles/yr before this motor is ready for an overhaul - in about 2023 or so.
Use premium gas if the engine's compression ratio is 10:1 or higher and
change the engine oil and filter every 7,500 miles. Regular gas
preignites (knocks), especially in hot weather and high loads, so the
computer retards the ignition to reduce the combustion temperature and
efficiency (mpg) falls.
The greater the demand put on the motor the more it needs premium gas.
You might try mid-grade during the winter and switch to premium in the