I agree with Dr. T--it's a combination of base-oil viscosity AND quality and VI additives. Thicker base oils have higher H-T shear strength, LESS VI improver generates higher ratings, higher-quality base oil has higher HTHS, and I suppose there's some additive that improves HTHS. I believe the presence of (more?) VI improvers in Castrol 0-40 versus 5-40 causes the significant reduction Dr. T quoted, altho the presence of some VI improver in Red Line's 5W-40 versus none in their 10W-40 affects their HTHS quality virtually not at all. (The 0.1 decrease from that of the latter to that of the former is only 2% and is statistically insignificant.) Maybe RL is using a VI improver with higher HTHS compared with the VII others use.