To point #1, once you get the engine warm, you want to maintain viscosity, and the difference between one part and another of an engine is not significant viscosity wise. To point #2, that's not exactly what was said. The dino has VI improvers to keep it from thinning out too much at high temps (and flow improvers for low temps). The Synth has flow point improvers (or simply different synthetics to flow better). so once a dino shears out, or loses it's polymers, it is a thin oil. If a synth lost it's flow improvers (don't think that can happen) it would still be the thicker oil.