In my truck, I ran M1 5w30 and my UOA showed that it sheared down to a 20 weight by the time I had changed the oil. Then, I ran M1 0w30 and the oil stayed within spec for 30 weight oil even after the same number of miles on the oil which was roughly 6500 miles. The additive packages appear to very similar with both oils. Also, I have heard on here by different people that the 15w40 Rotella is more shear resistant than the 5w40 version of Rotella. So what determines whether or not an oil can hold its grade?