I've linked to this in another post, which is probably bad practice, but I'm thinking if it hasn't already been discussed on here (If so I missed it) then this might be a better place for it.
http://www.api.org/~/media/Files/Certification/Engine-Oil-Diesel/Publications/4-Trends-in-Shear-Stability-of-Automotive-Engine-Oils.pdf
Its a presentation, so a bit short on detail, but they seem to be saying that viscosity for quite a lot of multi's increases after shearing.
I know it isn't news that viscosity starts to increase with degraded oil due to polymerisation of oxidation products, but they seem to talking about something new involving VI's.
"This behavior of viscosity increase with shear was observed primarily on 5W-30 oils, and primarily those purchased in Europe.
This trend has been growing over the last several years and is spreading to the other regions of the world."
Of one of the other papers mentioned, they say "VM chemistry does play a role in the viscosity increase phenomenon, although the authors were not sure why."
(Low-Temperature Rheology of Engine Lubricants Subjected to Mechanical Shear: Viscosity Modifier Effects, M. Covitch, J. Weiss, I. Kreutzer;Lubrication Science 11-4, August 1999).
Anyway, thought it was a bit surprising. Straight weight oil is looking better all the time.
http://www.api.org/~/media/Files/Certification/Engine-Oil-Diesel/Publications/4-Trends-in-Shear-Stability-of-Automotive-Engine-Oils.pdf
Its a presentation, so a bit short on detail, but they seem to be saying that viscosity for quite a lot of multi's increases after shearing.
I know it isn't news that viscosity starts to increase with degraded oil due to polymerisation of oxidation products, but they seem to talking about something new involving VI's.
"This behavior of viscosity increase with shear was observed primarily on 5W-30 oils, and primarily those purchased in Europe.
This trend has been growing over the last several years and is spreading to the other regions of the world."
Of one of the other papers mentioned, they say "VM chemistry does play a role in the viscosity increase phenomenon, although the authors were not sure why."
(Low-Temperature Rheology of Engine Lubricants Subjected to Mechanical Shear: Viscosity Modifier Effects, M. Covitch, J. Weiss, I. Kreutzer;Lubrication Science 11-4, August 1999).
Anyway, thought it was a bit surprising. Straight weight oil is looking better all the time.