Infineum Comments on HTHS Limits and its Impact on Oil Drain Interval

How HTHS viscosity at 150C relates to Viscosity at 100C. One idea I have is rather than solely considering the TBN as an indication of when to change the oil, one should also consider what the HTHS viscosity is of their used oil and make sure that it is still above the manufaturer's or specification's min. requirement, like Porsche and the ACEA with a min. of 3.5 cSt.

Here is the basic summary of the PDF:

Both KV100 loss and HTHS loss after shear are related to one
another as well as to the SSI of the viscosity modifier, with an average 32.2% of the KV100 viscosity being the after shear HTHS viscosity.

R^2=0.941 for % HTHS vis loss vs. KV 100C loss for 5w30, 10w30, and 15w40, so the data correlates well.

KV100 Stay-in-grade min. of 9.3 cSt = after shear HTHS min 3.0
15w40 after shear HTHS of 3.7 cP min recommended, so KV100 min should be 11.47 cSt in UOA.

That would suggest that if the oil in a used oil analysis had 11.47 cSt Viscosity at 100C or less, that it is time to change the oil.