You can't label a 15W oil as 5W, you have to label an oil with the lowest winter label it achieves, ad packs will be very similar, since they're both nominally 40 grade oils at operating temperature they'll both be about the same viscosity at 100C which is the only viscosity tested on a standard VOA, you can't determine the winter rating unless you run a cold cranking simulation or mini rotary viscosimeter test on the oil. The 15W40 is likely to use a heavier base oil and less VIIs and be more sheer stable than the 5W40 and likely to retain more viscosity in a UOA and maintain a higher HTHS viscosity throughout the OCIViscosity is about the same as are the additives. Is Shell pulling a fast one? Same oil but labeled differently for marketing purposes?