Originally Posted By: threeputtpar
I've been researching what oil to add to my free G-Oil to get the HT/HS number up from 3.2 to 3.5 for my Audi, and I've decided to go with 1 quart of RL 20W-50 to 6 quarts of G-Oil. It theoretically should be around 3.6 so that would suffice.
The question that I had then is why does an oil like RL 20W-50 have an HT/HS number of 6.0 and say M1 15W-50 only have 4.5? What is in the RL 50 weight that increases the HT/HS so much more? Thanks for the answers.
HTHS refers to high-temperature, high-shear.
Shear here refers to the shear rate of the closely and rapidly sliding parts and is equal to the speed of the sliding parts divided by the thickness of the oil layer between them, the resulting unit being 1/second.
Shear rate = (Speed of the sliding parts) / (thickness of the oil layer between the sliding parts)
HTHS viscosity is a dynamic viscosity in that the viscosity measurements are performed at a shear rate of 1,000,000 1/second. At such a high-shear rate of 1 million inverse seconds, the viscosity-index improvers (VIIs), which are polymers, temporarily shear by having their microscopic shapes changed (regaining their molecular shapes at low shear, not to be confused with permanent shear of VII polymers, which results in a permanent loss of viscosity, even after the shear rate is reduced). The result is a lower viscosity than you would see if you did the same test at a low shear rate, such as when you would be measuring the kinematic viscosities at 40 C and 100 C. (Also note that the units for HTHS and kinematic viscosities are different, cSt and cP, respectively).
Therefore, for a given base oil, the more the VIIs added, the smaller will be the HTHS viscosity in comparison to the kinematic viscosity. Therefore, if you have two oils with the same 100 C kinematic viscosity, if one of them has a higher HTHS viscosity, it means that it has less viscosity-index improvers added in the mix. In this case, Redline 20W-50 has less viscosity-index improvers than Mobil 1 15W-50, which makes sense because for similar base oils, the more the SAE viscosity spread, the more is the concentration of the viscosity-index improvers.
As a footnote, the more the concentration of viscosity-index improvers, the more will be the permanent oil shear, as the permanent oil shear is determined by the concentration of VIIs that permanently shear (concentration of polymer molecules that permanently change their shapes), which depends on (1) the overall concentration of VIIs and (2) the quality of VIIs.
