There may be some argument to be had regarding pressure and such with a lower vis oil. Compared to 15w40 in my Detroit, the 10w30 I am using only dropped 2 PSI across the spectrum. And iron, lead, tin, chromium, aluminum, etc are all the same or lower with the 10w30. So far lower, but only by about a few points. Nothing earth shattering. But the 10w30 is holding viscosity loss over time much better. And TBN depletion is not as much over similar time frames compared to 15w40. Same brand of both oils, both syn blend, same test lab, so comparison sound.