Multi-grade engine oils can be confusing. The nomenclatures confuses many people. And I don't know if what I'm about to tell you will make it any easier...but I'm going to say it anyway. All multi-grade oils: 0W-16, 0W-20, 0W-30, 5W-20, 5W-30, 5W-40, 5W-50, 10W-30, 10W-40, 15W-40, 15W-50, 20W-50, etc... Are all made in the same fashion. The proportions, base oils, and additives change, but the process is the same. You take a base oil that is design to meet you W rating, in this case a 5W, and you add viscosity index improvers (viscosity modifiers) that allow the oil to thin less as it heats up. Giving it the characteristics of a higher grade/weight/viscosity oil at operation temperature. In this case, an SAE 40 oil. It still gets thinner as it heats up, but it only gets as thin as an SAE 40, instead of getting as thin as a 5W, at operating temperature.

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