The current generation engines, being compact are running at very high temperatures. Measuring the viscosity at 100°C was not too representative of the temperatures reached today by the latest generation of engines. Therefore a standard test was introduced to measure viscosity at 150 °C, and under a high shear rate, so as to really examine the ability of the oil. Hence what HTHS (High Temperature, High Shear) viscosity stands for, the “mPa.s” is a unit to measure HTHS viscosity.
The concept exists of High HTHS and Low HTHS: A low HTHS oil has a lower viscosity at high temperature (<3.5 mPa.s) than that of a so-called High HTHS oil (>3.5 mPa.s). This notion is important as it allows for the classification, on the one hand, of fuel economy oils having a low HTHS and, on the other hand, of high protection oils having a high HTHS.
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