All oils use viscosity modifiers. As oil shears, two things happen:
1) The oil loses its ability to lubricate properly. Given enough time, the oil will lose just about all its lubrication ability. Oil that is sheared down from 40w to 30w will not lubricate as well as oil that is fresh 30w.
2) As oil shears, the viscosity additives break down, becoming contaminants that develop into sludge, varnish, etc.
There is waaay more to it, but if you want to learn more about shear and oil in general, go here:
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