What's the difference between Euro-spec oils like 0W-40 and standard 5W-30?

Euro-spec oils (ACEA A3/B4, VW 502/505, MB 229.5, Porsche A40, etc.) typically run thicker at operating temperature with higher HTHS values and stronger additive packages aimed at turbocharged direct-injection engines and longer European service intervals. Reviewers consistently warn against substituting them in non-Euro engines that call for a thinner grade — match the spec, not the brand.