As the VI improver breaks down it also gets too thin.
Does a VI improver always work in this direction as it breaks down, i.e., gets too thin? Or are there improvers that can get too thick when they break down?
Added by edit: I just read that, "The addition of modifiers will only slow down the rate at which the viscosity decreases." here, so I think that answers it.
Multi-grade oils start with thin oil base to which polymers [VI improvers] are added that thicken the oil at higher temps. So, when these polymer additives deteriorate due to mechanical action or oxidation, oil becomes thin.
That is a good explanation of multi-grade oils. I like to explain it this way. All oils will "thin out" as they are warmed up. The difference with multi-grade oils is with the addition of viscosity index (VI) improvers multi-grade oils will "thin out" at a much slower rate as the temperature increases. Think of it as the slope of a graph of viscosity vs. temperature. So, a 10w30 oil will act like 10 grade oil when cold (making cold starts easier) and like a 30 grade oil at high temperatures (thus protecting the engine). Hope this helps.