Standard oils follow a mostly linear curve when you measure viscosity versus temperature. As you mentioned, they get thinner with temperature. Multi-weight oils also get thinner with temperature, but they contain polymer chains that twist as they heat up, which make them behave like a heavier oil. This "bends" the viscosity/temperature chart. It still gets thinner as the oil gets warmer, but not as quickly as standard oil.
You're spot on that the 10w-40w will behave like a SAE10w oil at low temperatures, and then more like a SAE40w oil at operating temperature, however 40w oil at ~200degF is still thinner and more fluid than 10w oil at 40degF. It's just not as thin as 10w oil would be at ~200degF.
That said, by running the engine until warm, you're still thinning it down to make it drain easier and bring particulate into suspension so it can be swept out...you're just not getting it as thin as it would be if it were single-grade oil.
Reply With Quote