Don't worry about what I said about emotions. I get emotional, too. I know professors in my department who got into fist fights over science at conferences. At least we are not that bad.
What the second Nigel Marx, Hugh Spikes, et al. paper says about increasing the temporary shear (temporary shear thinning) to improve the fuel economy in your excerpt above, is summarized in one variable in my table: A_Harman index.
1 - A_Harman index is the temporary shear (thinning). So, when the A_Harman index decreases, the temporary shear increases, and the fuel economy improves. Of course, VII (effective content,
not actual content) = (1 - A_Harman index)/2; so, the VII variable also gives you the temporary shear, divided by a factor of 2.
Increasing the viscosity index VI of the finished oil also improves the fuel economy because the oil runs thinner over a wider temperature range. For example the HTHS is specified at 150 °C, but the oil temperature is usually much lower, and a higher-VI oil with the same HTHS will run thinner than a lower-VI oil at most operating temperatures.
Other than the 1 - A_Harman index (temporary shear) and viscosity index VI, the HTHS plays a major role in fuel economy because it is related to the hydrodynamic friction in the bearings.
This all comes at a cost. Decreasing the A_Harman index decreases the base-oil viscosity HTFS, which potentially increases engine wear. Moreover, the VII content increases, which potentially increases engine deposits. The permanent shear will increase, too, which may be a concern in viscosity-critical applications.
By the way I prefer a higher A_Harman index, which gives a higher base-oil viscosity HTFS, to be on the safe side with regard to engine wear. Higher fuel economy is good, too, especially in a car like mine that averages around 65 mpg (3.6 L / 100 km) with regular gasoline in mixed driving and you want to hit the 100 mpg etc. mark in certain driving conditions, like in slow traffic. At the end of the day, I am stuck with TGMO 0W-16 at the dealer, and when the free maintenance expires, I may go with Mobil 1 AFE 0W-16.
On a different subject, I noticed something very unusual in Table 2 of the first paper yesterday. Look at oil #'s 11-13. For a fixed finished-oil HTHS, a fixed base-oil KV100, and a fixed DDI package, the
actual VII content (
not the VII variable equal to the temporary shear thinning divided by 2 in my calculator) is higher for a Group III base oil than for a Group II base oil and higher for a Group IV base oil than for a Group III base oil.
This goes entirely against the intuition of everyone on BITOG including me about synthetic oils having less VII than conventional oils! It has to do with the variation of the thickening power with temperature for different base-oil types, which is given in Figure 4. The higher the viscosity index VI of the base oil, the flatter the curve is. It is very interesting. However, other VII's may not behave in this fashion, and this particular star VII may be exceptional with its increasing thickening power with temperature. Since the comb PMA VII has a curve similar the curve of this particular star VII, it may also have a similar dependence on the base-oil type or viscosity, which wasn't studied in the paper.