Manufacturers specify both normal and severe service intervals, but they never describe the difference in any usefully specific way. Your normal could be more or less severe than their idea of normal. So as your viscosity additives break down, your 5w40 becomes 5w35, becomes 5w30, etc, until you change it out whenever. Used oil analysis will tell you the actual viscosity along with the degree of contamination. You could find out if your chosen interval is shorter than it needs to be, or if you are pushing it too far. Either way, that's good to know. With an older motor you can identify wear byproducts like bearing or ring metal before you would observe any symptoms otherwise. Again, good to know if a major overhaul is in the near future.