I don't know if I'm answering the question that was asked, but I'll give it a try. The viscosity properties of a solution of polymers is largely determined by the longest (or largest) molecules in the mixture. So if you mix a 10 weight oil with a 30 weight, you don't end up with a 20 weigth oil, but rather a mixture with closer to 30 weight viscosity characteristics. The largest molecules affect the viscosity at both 40 degrees C and 100 degrees C. This holds for straight weight oils, which exhibit Newtonian viscometric properties, which are determined just by polymer length. One way to look at this is that longer polymers have larger surface areas that can form a greater number of weak chemical interactions with other molecules than shorter polymers. That doesn't really explain the whole viscosity thing, but it is a start. When VI improvers are added to a base oil, they "dissolve" in the base oil since they are themselves hydrocarbons that can form the same types of weak interactions with the hydrocarbons in the base oil as the base oil components do with themselves.
Heat is just molecular motion. As the amount of heat in a system increases, the interactive forces between molecules will decrease and viscosity of a solution goes down. VI improvers increase in size as the temperature goes up either by uncoiling or expanding (depending on the type of additive) as the temperature increases. Thus, thus with increasing temperature, a certain percentage of the molecules in a multi grade oil get larger and expose more surface area for interactions with other molecules. This tends to counteract the effect of heat on the base oil molecules and has the effect of minimizing the effect of temperature on viscosity.
This is an oversimplification, since the shapes and chemical properties of the interacting molecules themselves also influence viscosity, especially when shear forces are acting on the solution. But I hope this helps.
