Originally Posted by Patman
You're forgetting the fact that the 5w30 will have a higher HTHS of around 3.0 vs 2.7 for the 5w20, and even when it thins out a bit it'll still have a stronger HTHS than 5w20.
I can't undersant talest's logic. Sure a 5w30 or 0w30 might get permanent shear at some point, usually way later toward the end of the oil change interval anyway.
It depends on the application, the conditions which produce more mechanical shear, shredding of the VII chemicals, with some oxidation breakdown over time. The Honda S2000 high-RPM engine comes to mind as something that shears a lot of oils. New engines with freshly machined surfaces are responsible for more shear in my estimation.
What is seen most of the time is any multi-visc oil that has VII will gradually get permanent shear
ramping in,
not all of a sudden of course, over the first 3,000 miles, and then oxidation of the oil itself will gradually raise KV100 back up.
I think I've seen Mobil's viscosity dip in engine oil UOAs. See graph below. Not a big dip usually. (Of course fuel dilution can really make visc dip down, but that's another effect not everybody has either.)
Notice oxidation effects can begin at some point, raising hot kv100 viscosity over time.