How’s does vis 100C shear compare with HTHS shear?

[Klutch9]

Let’s say I have a 0w20. Starts its life at 8.4 cst, and a UOA shows that it’s down to 7.8 cst by the end of the OCI. This is purely hypothetical of course. But say that oil starts at an HTHS of 2.6. How does its shear/viscosity loss correlate with the vis 100C value? Is it linear? Perhaps it depends on fuel dilution/actual shear. Just something I’ve never thought of before.

 

[Jetronic]

It's not linear. The part of the oil that shears and is responsible for the viscosity loss at 100°C is the viscosity modifier. The HTHS viscosity is measuring the oil while shearing. So now the 2 viscosities are closer than for new oil. Typically, the HTHS suffers half the %-age loss of the kv100, so in your example 7% KV100 loss means an expected 3.5% HTHS loss.

If the viscosity drop is due to fuel dilution, the loss is more linear.

 

[userfriendly]

Noack, evaporation losses also counter thinning from permanent shear.

 

[ZeeOSix]

There's an easy way to mitigate viscosity loss when starting near the bottom to begin with.