Trying to make absolutely certain I understand this correctly. Oils with large viscosity spreads (5w50, for example) require very good basestocks and/or VIIs. And in general, those VIIs will lead to shearing under extreme conditions, such as sustained WOT. This is because the VIIs breakdown under load. Conversely, oils with small viscosity spreads (5w20, for example) do not require as good of a basestock and/or fewer VIIs. Since there is less reliance on VIIs, the oil is generally more shear stable. Correct? Trying to get clarity as the weather warms here and racing season begins for my DD. Thinking a move to 10w40 might be more prudent now, and leave the 5w30/5w40 for the winter. HTHS is a very important number for racing conditions, but shearing is still a concern. The 10w40 I am considering has a slightly lower HTHS and VI than the 5w40 (4.19 vs 4.51, and 161 vs 196, respectively), but I would assume it is much more shear stable oil and better suited to racing.