I haven't done any research on this but i have heard that a straight weight oil of 40W or higher the additives are different. That is they are similar to the additives that were in oils of just a few years ago. I guess it's because no OEM specifies anything but oil like 0W-30, or 5W-30, or 10W-30. So if you use a 40W oil it's for racing or old engines.