I cant figure out how,... on what I know about motor oil, how this Amsoil series 3000 5w30 can perform supposedly so well in a turbo diesel engine which stresses oil so much more than a gasoline given the fact that not only do you have very very high loads on main bearings from power/torque, but you have on "paper at least" less protection as far as HT/HS,and viscosity, compared to a 15w40 oil "synthetic or not" which in my mind mean less protection on highly loaded crankshaft journals? Is this oil completely loaded with ZDDP unlike the newest CJ-4 spec oils? Even if so...you still have the heat from the turbo-charger to break this 5w30 oil down even more! What makes this oil such a "phantom oil" over a 15w40? Amsoil even recommends this oil over their 15w40 AME in most applications prior to 2007 turbo diesel trucks. I see that CST is on the higher end of a 30wt oil...but still it is what it is! NOT a 15w40! Chime in all! especially user's of this oil with UOA's! -Ponch