Keith,

Great post. The viscosity of the multigrade oil is measured once at 0&deg F and again at 210&deg F. The idea to remember is that when the oil is measured at 210&deg and comes out as a 30 weight (your example), that this oil didn't get "thicker", it is merely measured on a scale calibrated for 210&deg.

If a straight 30w oil was measured at this same temp with this same scale, it would measure as maybe a 10 weight oil.

So the gist of this is that with VI improvers, the oil doesn't get thicker at elevated temps....it just doesn't thin out like a single grade.

Stuart