An easy read mini tutorial on oil viscosity index shearing by Chevron Oronite. https://www.oronite.com/paratone/shearloss.aspx

Viscosity Index Improvers (VII) are used in formulating our multi-viscosity engine oils. The type choice of VII's is important in making a recipe's VI stable, so it won't change much from heat, pounding, squeeze, shear & fuel dilution, during an OCI.

Heavy molecular weight/large size molecule VII's are more prone to Permanent shear/tear viscosity loss, than lighter weight/smaller size molecule VII's.

So the formulators choice of VII's for our particular engine oil recipe & our particular engine design, will impact how much viscosity drop/thinning is due to mechanical molecular streaching/temporary viscosity loss, or molecular tearing/permanent viscosity loss.

Squeeze, shear & heat figure in on how easily or how bad the shear is & if its just temporary, from the VII molecule being streached, or permanent, from the molecule being torn, in which case the oils viscosity will drop/get thinner & stay thinner.

Used Oil Analisis (UOA) with fuel dilution testing, will tell us what the used oil viscosity is & if it has dropped, is it due to fuel dilution/thinning, or mechanical shear/tearing of the VII molecule & that way we can determine if our oil choice is up to the task of properly lubing our particular engine design, the way we're using it during that OCI.

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