I have been using PP 5-30 and Valvoline Synpower 5-30 in the car since new, the odometer reads 12,000. My highway fuel mileage typically is about 25-25.5 at 75 mph with the AC on with 5-30. Last change I switched to Rotella T 5-40, my highway mileage dropped to about 22.5 to 23. Is this pretty...