First, the wider a split between the cold (5w) and hot (40) viscosity ratings, the larger and/or great quantity the viscosity modifiers need to be. The greater the amount of additives (friction modifiers, viscosity index improvers
("VII"), detergents, anti-foaming agents, etc) you add to oil, the less base oil you have in the mix, and the more rapid the breakdown of the oil through shear, coking, etc.
The larger the viscosity improvers are, the greater the temporary shear thinning.
Especially with non-synthetic bases (including some ultra-refined petroleum oils masquerading under the term "synthetic"), there are possible severe adverse effects from the index modifiers. 10w30 has a 3x split, and is stable. 20w50, 2.5x split. 5w40, 8x split. So if you really feel the need to run 5w40, make it the best. Mobil1, etc.
Here's a summary article that explains a lot (page sponsored by amsoil):
What Is Oil Viscosity Part 2: Oil Viscosity Grades
www.selectsynthetics.com
some quotes:
"In other words, what this means is that a 30 grade oil has pretty close to an ideal viscosity (around 10 cSt) at operating temperatures but is way too thick when cold (around 250 cSt) whereas a 10W grade oil has a more acceptable/desirable viscosity (around 30 cSt) when cold but is way too thin (around 5 cSt) at operating temperatures.
* Again,
kinematic viscosity is
the amount of time, in centistokes (mm2/s), that it takes for a specified volume of lubricant to flow, under the force of gravity, through a fixed diameter orifice at a given temperature. "
and
"
To make sure the VII is used in the most cost-effective way,
polymer thickening efficiency is also important. Thickening Efficiency (TE) describes the boost in Kinematic Viscosity at 100°C of an oil following the addition of a specific amount of polymer. A polymer having a high TE indicates that it is a potent thickener. TE is primarily a function of polymer chemistry and molecular weight.
The downside to
mineral-based multigrade oils is that they require a lot more VII in order to meet the proper viscosity requirements. Problem is, the VII additive wears out (shears) over time, effectively reducing the oil’s viscosity, until the oil becomes too thin to provide adequate protection (which is one of the reasons it needs to be changed more often than synthetic oil)."
and
"
Oil experiences very high stresses in certain areas of the engine such as in the oil pump, cam shaft area, piston rings, and any other area where two mating surfaces squeeze the oil film out momentarily.
An oil’s viscosity can be severely affected when an engine is exposed to high temperatures and the high shearing forces that are created when an engine is operating under load. These high temperatures and shearing forces can cause an oil to thin out and lose its load carrying ability, resulting in engine wear."
and
"
As oil is squeezed between two mating surfaces, such as between a bearing and a journal, VII polymer molecules, or coils, have a tendency to align with each other and get “squashed”. This causes the polymer coil to deform (elongate/stretch) and become aligned to the direction of flow.
When this happens, viscosity
temporarily drops resulting in oil film thickness reduction. After the oil progresses through the bearing, the polymer coils “spring back” to their original shape and viscosity returns to normal. This phenomenon is referred to as
temporary shear-thinning.
In certain high shear conditions, the long flexible polymer chains can be ruptured or pulled/ripped apart (molecular scission) into smaller chains. When the shearing force is removed, the broken polymer chains cannot re-form into the single large chain because the coil has been physically and chemically changed.
Consequently, this causes the oil to
permanently lose viscosity leading to a reduction of oil film thickness which can lead to oil film failure and an increase in engine wear. For example, an SAE 5W-30 oil can ‘shear back’ to an SAE 5W-20 oil or worse, resulting in damaging deposits, increased wear, and reduced engine life. This phenomenon is referred to as
permanent shear-thinning."
