One of the specs that I look for in an oil is that it meets not only the A3/B3, but also the B4 which is a tougher diesel spec than B3. While looking at a bottle of M1 15w-50 I noticed it meets the A3/B3 spec but did not list the B4. How can a thick oil like this not hold up to the B4 spec when a thin oil like GC 0w-30 can? Even if the M1 has alot of polymer VI's, would it shear to a weight less than GC? I wouldn't think so, but I guess it would. So what am I missing here?