quote:
Originally posted by Jay:
I'll try. Shear stability has two meanings: there is a temporary shear that occurs when oil is under stress--the oil actually squishes and thins under compression, but recovers when the stress is released. Then there is permanent shear that occurs when the long-chain molecules are actually cut, and the oil permanently thins.
Oils with very high viscosity indexes generally, but not always, tend to be more prone to temporary and permanent shearing than oils with low VIs.
All things being equal, a 0w-30 will be thinner than a 10w-30 in actual use in an engine. This is certainly true of M1 0w-30(HTHS 2.99cP) vs 10w-30 (HTHS 3.17cP). If you could accurately measure it, you'd see slightly better gas mileage with the 0w-30, even though the 0w-30 has slightly higher kinematic viscosity than the 10w-30. 2.99cP represents less viscous drag than 3.17cP.
Is M1 0w-30 less shear-stable (in regards to permanent shearing) than M1 10w-30? Possibly, I don't know. They're both very shear-stable oils. Is M1 0w-40 (HTHS 3.6cP?) more shear-stable than M1 0w-20 (HTHS 2.6cP)? A thousand times no!