What prevent oil from shearing down? Additives?

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Oct 21, 2003
Maryland, USA
Many members here claim very long OCI beyond 3K miles with synthetic oil. If the UOA shows good result after an extended OCI, does this mean the oil is not shearing down? Based on what I read on bypass filter the oil can be reused over and over again with proper filtering. What prevent oil from shearing down? The additives? But they do wear out, right?
I'm not as educated on this topic as many other members, but I'll give it a shot. The main factor that makes synthetics more shear stable than dino oils is the superior base oil itself. True synthetics have a naturally higher viscosity index and do not require the amount of polymer index improvers that dino oils need to achieve a multigrade range. As a dino oil is stressed over time, those polymers can 'burn off' and lead to shearing.
It most cases the polymers used as viscosity improvers are more prone to shearing than the base oil. Because synthetic oils can meet the multiviscosity rating with little or no viscosity improvers they're less likely to shear down ovewr time.
Thanks for the replies. Now I understand the need for the annual oil change for an engine running with a bypass filter and synthetic oil.
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