Originally Posted By: BobFout
Originally Posted By: Gokhan
Originally Posted By: BobFout
I find it odd the 5w-30 and 10w-30 have the same 100C visco with such different HTHS visco.
Not odd at all, on the contrary, it makes perfect sense. 5W-30 has more VII, therefore has a lower high-shear viscosity, as VII temporarily shears in high-shear conditions. Of course, VII could also permanently shear and reduce the low-shear 100 C viscosity as well.
Typically to get such a low HTHS, the 100C visco is also quite a bit lower, in the 10 or 11 cSt range. Reference regular Mobil 1 5w30 (11 cSt and 3.1 cP HTHS) and 10w30 (10.1 cSt and 3.0 cP) . I'm betting it's a documentation error.
Nope.
In order to understand this, you need to understand how VII (viscosity-index improver) and HTHS (high-temperature, high-shear) viscosity work.
VII improves HT (high-temperature) viscosity by extension of the polymer molecule. However, these molecules also temporarily shear under HS (high-shear) conditions, leading to a loss in viscosity. 40 C and 100 C viscosities are measure under LS (low-shear) conditions, where VII molecules don't shear. Note that shear refers to how fast the two engine parts are sliding and how close they are -- the ratio of their relative speed to the oil-film thickness. The shear rate used to measure and report HTHS viscosity is 1 million per second, meaning that if the oil-film thickness is 1 micron, the two parts slide for 1 meter in 1 second.
All this means is that M1 HM 5W-30 uses more VII than typical. This large concentration of VII results in a strong 100 C low-shear viscosity but a weak 150 C high-shear viscosity. You can estimate amount of VII in an oil by comparing the ratio of the 100 C low-shear viscosity to HTHS viscosity to those of other oils with the same SAE viscosity grade, similar base oils, and similar VII molecule. Higher the ratio is, higher the VII concentration is.
Note that this is exactly why older cars do not recommend 5W-30 while they recommend 10W-30. Even though 5W-30 and 10W-30 have the same 100 C low-shear viscosity, 10W-30 has a stronger high-shear viscosity (HTHS viscosity) because it contains less viscosity-index improver and you don't have to worry about temporary shear in high-shear areas (sliding areas) of the engine.
Another. more widely known, concern, something people mention a lot here, is the permanent shear, where VII molecules are permanently broken. This happens as oil ages (typically a few thousand miles) and both the low-shear and high-shear viscosities are permanently reduced in used oil due to the permanent breakdown of the VII molecules.
Some more reading, albeit marketing-oriented, is here:
https://www.oronite.com/paratone/shearloss.aspx
