oil does not wear out, modern oils are designed to run up to 6000 miles in modern engine, with much more rpms and combustion pressures than the A will ever achive, 2500 miles is a reasonable amount of miles to put on an engine before an oil change, and contrary to another post multi grade oil does not contain any type of acid, its the combustion process that causes acid, that's the reason for oil changes, modern engines run 300000 to 400000 thousand miles with out any acid caused by the oil, an old(new) wives tale, in closing, if the oil starts to look dark its picking up contaminats, and that is the time to change, and to further aggrevate the some on here, detergents do not hurt or dissolve babbit, the thin coating on modern bearingsi s babbit and nothing happens to it even over thousands of miles of driving other than just pure wear