As I understand it, engine oil becomes more acidic over time. Oil manufacturers now make the oil more alkaline to neutralize the ph for most of the life of the oil. If you change you oil too often it causes you oil to be too alkaline and therfore more engine wear occurs.
In the past, before the changes in oil production, engines needed more frequent oil changes. Now, new cars recommend 7500 miles or more between changes. Of course, oil change shops recommend 3000 miles. It increases their profits.
