Viscosity improvers aren’t lubricants, therefore they reduce the lubricity of the oil. Synthetic base stocks behave much better in terms of viscosity change during temperature ranges. Synthetic is simply more slippery. It’s like most other things in life…..to get the first 90 percent of ‘quality’ is easy, it’s getting the last part that tougher. NASA developed them a long time ago, Mobil brought them to automotive marketplace.