Short answer
Shear stability is an oil’s resistance to permanent viscosity loss as the polymer viscosity index improvers in it are torn apart by mechanical stress. It is usually reported as percent loss of kinematic viscosity at 100 °C after a standard shear test, such as ASTM D6278 (30 passes through a diesel injector) or ASTM D7109 (30 and 90 passes). Many specifications require an oil to stay within its SAE grade after shearing.
Temporary and permanent shear
Long polymer molecules thicken multigrade oils at high temperature. Under fast shear in a bearing, they stretch and align and the oil thins temporarily; it recovers once the shear stops. HTHS viscosity measures that temporary effect.
Permanent shear is different: the polymer chains break, and the oil stays thinner. Gear contacts, pumps and injector-style flow are typical causes.
How shear stability is tested
The diesel injector tests force oil repeatedly through a nozzle and measure viscosity before and after. ASTM D6278 runs 30 passes; ASTM D7109 reports after both 30 and 90 passes, and its 30-pass results are not interchangeable with D6278’s.
European specifications also use a tapered roller bearing shear test (CEC L-45). Each specification states which test and which limit applies.
Where shear matters most
Motorcycles and some powersports engines share oil between the engine, transmission and clutch, so gears work the oil directly. Heavy-duty diesel engines with long drain intervals and high-pressure fuel systems also put more mechanical stress on the oil than a typical passenger car.
Oils formulated for those uses often use more shear-stable polymers or less polymer overall, relying more on base oil viscosity.
Spotting shear loss in service
Used-oil analysis that shows viscosity falling toward or below the grade can mean shear, but fuel dilution produces the same symptom. Labs separate the two by testing for fuel and looking at flash point and other indicators.
Frequently asked questions
Do straight-weight oils shear?
Monograde oils with little or no polymer have little to lose through permanent shear, although their base oil can still oxidize and change over time.
What does “stay in grade” mean?
That after the specified shear test, the oil’s 100 °C viscosity still falls within its SAE J300 range, so a 10W-40 still measures as a 40.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].