What the Heck is Shear Stability in Motor Oil

by admin on June 10, 2010

There is a lot of technical jargon in the motor oil industry and somehow these guys think people understand what they are saying when they talk about things like “shear stability”.  I find that people are more interested in hearing about more horsepower and better engine protection then they are in hearing about technical details like shear stability.  The question is how does shear stability relate to more horsepower and better engine protection?  Great question.

I am going to get a little abstract here to help understand what is going on at a molecular level.  Motor oils get thinner as they get hot.  In a conventional oil the oil gets so thin when it heats up it will not protect an engine properly.  To prevent the oil from getting too thin a VI (viscosity improver) is added.  The VI improver keeps the oil “thicker” at higher temperatures to better protect the engine… but here is the rub.

If you could zoom in and look at motor oil with an electron microscope you could visualize VI as coils of spaghetti.  When the VI is cold the spaghetti coils are very compact and bounce around and move around each other easily.  As the VI improver heats up the coils elongate and unwind and start clogging up on each other.  This makes the fluid move around less freely and counteracts the thinning of the base oil.

If cheap VI improvers are used the oil will wear out faster in your engine.  The shearing effects of the moving parts in the engine rip the VI improver apart and you end up with oil thinning.  All the little spaghetti molecules are broken into little pieces allowing the oil to flow to easily at high temperature.

This could mean a little bit of cheap horsepower with the terrible cost of loss of engine protection…  more engine wear.

Shear stability of an oil is an indication of how long the oil will maintain its protection due to viscosity.  If an oil has low shear stability then it probably won’t protect long.  If a motor oil has high shear stability it has a good chance of protecting your engine longer.

If you would like to see a comparison of the top oils and where they fall on the shear stability chart then you can check them out here with the best motor oil review.

Hope that helps!


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