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In order to calculate the viscosity index of a lubricant, apparently you need two input parameters. The kinematic viscosity (mm²/s) at 40°C and at 100°C.

If I got it right, mm²/s means the distance in square millimeters a fluid can travel through a capillary within the timeframe of a second.

Usually, most materials are "more fluid" when heated, which should imply that the viscosity will be lower, the higher the temperature. As far as I know viscosity is the opposite of fluidity => honey is more viscous than water.

So this is my assumption:

  • More mm²/s traveled = Lower Viscosity
  • Less mm²/s traveled = Higher Viscosity

But now, everywhere I looked online (see e.G. links below), the input value of V40 has to be higher than V100.

Does that mean oil can travel less distances through a capillary when it's hotter?

Please be so kind to explain in layman terms as I'm not too well versed in the field of natural sciences

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  • $\begingroup$ are you sure it is 40 and 100 degrees, or -40 and -100 degrees, -100 is colder than -40 degrees $\endgroup$ – Adrian Howard Oct 16 at 18:07
  • $\begingroup$ @AdrianHoward I don't think so. According to wiki.anton-paar.com/en/viscosity-index [...] Using these parameters and the measured viscosity of the oils at 40 °C and 100 °C, the viscosity index can be calculated $\endgroup$ – Tom M Oct 16 at 18:12
  • $\begingroup$ Viscosity of liquids decrease with increasing temperature. $\endgroup$ – Chet Miller Oct 16 at 18:32
  • $\begingroup$ @ChetMiller which is the reason I asked this question. Probably the title is not descriptive enough $\endgroup$ – Tom M Oct 16 at 19:38
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    $\begingroup$ Yes, that is the case. A higher numerical value for the viscosity means a higher viscosity. Maybe the problem is in the units, you are calculating the kinematic viscosity but I think the dynamic viscosity is easier to interpret: The units are $Pa\cdot s = N \cdot s /m^2$, it (kind of) gives the opposing force over the area of contact (shear stress). $\endgroup$ – S V Oct 16 at 23:11

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