Is there a certain High Temperature/High Sheer viscosity that is important to the life of your engine? That is, if I put an oil in my engine that has a low HT/HS, even though it is the correct grade (i.e., 100C kinematic viscosity is appropriate), will I risk significantly greater wear? Or is HT/HS a non issue?

In particular I am concerned with my motorhome that weighs 10,000 lbs and has a huge 7.5 liter V8 engine, but also in general would like to consider the HT/HS questions for passenger cars too, some of which are driven hard.

Also, is HT/HS a good way to evaluate the sheer stability of an oil with respect to polymer sheer in the viscosity index improver additive?

Look at the SAE 300 Spec. If you use the correct kinematic viscoity, your oil must have the right HT/HS. You will risk your engine life in case you experience fuel dilution, high RPM and a HT/HS value < 2,5.

HT/HS is not really a good way to determine the stability of the VI's, although it is influenced by this.

Bearing clearance has been a recent issue

S = (r/c)^2* other stuff. r is bearing radius and c is bearing radial clearance. Note that r/c is squared.

with 3 other factors (Pressure/load, Apparant Viscosity in the bearing, and RPM. This number is then used in graphs/charts to specify other design parameters. However, this viscosity is not the stated viscosity of the oil at 100C it is inside the bearing. And a viscosity change or a clearance change would change the temperatures which would go back and change the viscosity inside the bearing (yet again).

Is the HTHS being worked back to a 100c vis to give a ballpark for the oil vis to use in normal road use.

However the HTHS test showed protection invreased with HTHS and Porsche specify 3.5.

There appears to be a trade off between normal road use (mpg saving) and "Porsche" use (expected thrashing) re HTHS.

Low vis oils with High HTHS may be the best solution but are expensive and avoided by oil companies with a few exceptions.

Going back to the equation - low vis oils for high rev Honda engines may be appropriate, however you do wonder about the Ford motive.

HTHS is probably more important as you move away from general road use.

What is HTHS of used mineral oil after 3k miles?