Ken,
I don't think any bench test does a very good job of predicting shear stability in an actual engine. The kind of thinning out I see is the result of cumulative damage over 3000-6000 miles and many heat & load cycles.
What you get out of the HT/HS test is an idea of how much polymer is blended into the basestock. For example, if two oils have the same kinematic viscosity @ 100C, but one has a significantly higher HT/HS viscosity @ 150C, you can conclude the thicker oil uses less polymer.
I have found for example that the PAO based synthetics will have a relatively better value for HT/HS viscosity compared to a Group III oil. The reason is that the VI of the Group III basestocks isn't as high, so more polymeric thickener is needed to make the various SAE grades. I have also seen more shearing of the Group III oils in actual service. The Petro Canada 5w-40 is an example of a Group III oil that will shear quite a bit - in contrast to say Delvac 1, 5w-40.
TooSlick
