Making a 0w-20, you start with a thinner basestock oil compared to a 5w-20, and the 0w-20 will drop in hot viscosity over time sooner than the 5w-20 since the 0w-20 relies on VII polymers more. This varies some with what oil, GroupII or GroupIII,IV etc we compare, but in general Honda got nervous about what might/could/does happen in some of their high-performance engines especially. GM does not look as nervous about it.
Chevron Oronite's website says it best: "The oil’s viscosity falls during the test due to polymer coil breakage. In other words, only that part of the oil’s viscosity which is contributed by the VII polymer is susceptible to breakage. Neither the base oil nor the additive performance package suffers permanent viscosity loss. Moreover, different VII polymers have different shear stability characteristics, depending on the molecular weight and chemical nature of each. Those VIIs having higher molecular weight have a greater propensity for polymer coil breakage.
A VII polymer’s “Shear Stability Index” (SSI) is defined as its resistance to mechanical degradation (polymer coil breakage) under shearing stress.
Example: An oil is formulated with base oil of viscosity 5 cSt and a VII is used to increase its viscosity to 15 cSt. The VII’s viscosity contribution is therefore 10 cSt. During the shear test, the oil’s viscosity falls to 12 cSt. It has permanently lost 3 cSt of viscosity.
VIIs are available across a range of the SSI and oil formulators choose the appropriate VII product that allows them to meet their finished oil performance and marketing needs. "
Making a 0w-20, you start with a thinner basestock oil compared to a 5w-20, and the 0w-20 will drop in hot viscosity over time sooner than the 5w-20 since the 0w-20 relies on VII polymers more. This varies some with what oil, GroupII or GroupIII,IV etc we compare, but in general Honda got nervous about what might/could/does happen in some of their high-performance engines especially. GM does not look as nervous about it.
