Measuring Shear Stability

[buster]

This was brought up in another thread. Why does M1 0w-40 have a 3.6 HT/HS and M1 10w-30 have a 3.2 yet 10w-30 won't shear down at all? We've mentioned this before that it's the VI's in the 0w-40 which makes sense but the 10w-30 never, ever has sheard down a bit. If this number is indicative of an oils ability to stay in grade, it's not making sense in this case. TS said something about this is not the only way to measure an oils shearing ability. I also here people say that a synthetic 10w-30 is equivelant to a dino 10w-40. Well even a dino 10w-40 can have a higher number then 3.2 so what does that really mean?

 

[Patman]

HTHS has nothing to do with whether or not an oil will thin out overall during your interval, but it has more to do with how much more the oil will thin out at 150C compared to it's viscosity at 100C. At least that's the way I understand it, it's something along those lines.