Do all 5W40 shear into a 30?

Joined
Jan 3, 2020
Messages
858
Location
Brittany
I've been looking at some UOAs and they all seem to end up around 11 to 12 cSt, even during a short OCI on standard naturally aspirated, port injected engines. To me it would only be a rare occurence caused by specific engine designs or fuel dillution in direct injection engines. Do all euro 5W40 have the tendency to shear into a 30 grade quickly? What are the causes? I've used this type of oil a lot but it looks like a euro 5W30 would do just as good and not shear as much since they have a thicker base and less VII.
 
Joined
Jun 4, 2003
Messages
1,557
Location
98245
As the oil is in service, its viscosity can change in either direction. Extreme pressure, like the cam-lifter boundary or some kinds of gear tooth profiles, can shear the oil's viscosity improving additives, gradually reducing viscosity. Other factors work in the opposite direction, like gradual oxidation and contaminant buildup which increase the oil's viscosity. The net effect - whether the viscosity goes up or down and by how much - depends on the oil and the application. In most cars I've seen the viscosity gradually goes down. But just cuz the viscosity stayed the same, doesn't necessarily mean the oil is still healthy. Maybe it stayed the same because it sheared down, but oxidation & contaminants thickened it. The full picture in UOA results can determine whether it's still healthy, not just viscosity alone.
 
Joined
Dec 26, 2005
Messages
22,972
Location
Upper Midwest
Depends on the oil, depends on the engine, depends on the use.
Exactly. It is highly dependent on the engine design. Very few engines have a problem with mechanical shear and most of the time a viscosity deviation is due to fuel dilution.

Plus Blackstone has issues measuring viscosity and most of the UOA on here are from them. Shear is a boogeyman that’s mostly a phantom.
 
Joined
Jun 4, 2003
Messages
1,557
Location
98245
... Shear is a boogeyman that’s mostly a phantom.
For cars, I agree it's the exception not the norm. For motorcycles where the engine oil also lubricates the transmission & multi-plate wet clutch, shear is more likely.
 

OM605

Thread starter
Joined
Jan 3, 2020
Messages
858
Location
Brittany
Thanks. I had doubts concerning Blackstone when it comes to viscosity. However, 30 grade oils seem to stay closer to their original 100c cSt most of the time.
 
Top