In the thread I linked to above Molakule posted various things that will affect HTHS viscosity. I understand your overal generalization, it's just that you're assuming facts not in evidence in this case, and ignoring facts that are.quote:
Originally posted by TooSlick:
...If you look at the VI's of "neat" PAO and Polyolester basestocks, about the highest VI you can get from a single basestock - usable for a multigrade engine oil - is about 145-155. However blends of two or more basestocks can achieve VI's >> 150. For example, the very thick PAO basestocks ("Supersyn" type) have VI's as high as 300. A small amount of this blended in can significantly improve the properties in the 40C to 100C range (significantly increasing the VI), with little degradation to the low temp CCS or cold pumping viscosity.
The gist of what I'm saying is that looking at a VI and a general class-distinction of base stock (i.e. base oil group "above" III versus "below" III) and deriving an HTHS number seems fairly off-base for both the reasons Molakule gave and the ones you give above. That combined with the Savant "verification" of RedLine's numbers and "dispute" of Amsoil's seem to be enough to support the idea that HTHS numbers are a little more variable than you imply.