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International Automotive Technicians Network
Re: Engine oil for older ag and industrial equipment
Posted to Technical Discussion Forum on 12/13/2023

Essentially correct.

Any early "SAE XW-XX" started with lower HTHS viscosity than a similar SAE XX.

Additionally (as you suggest) the Viscosity Improvers (VI additives) were also pretty fragile and would break down over time and use. Login to read more.