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# Viscosity of 15w-40 at room temperature2

## Viscosity of 15w-40 at room temperature

(OP)
Would someone be able to help me out and tell me what sort of range of viscosities 15w-40 could fall into at approximately 20 degrees centigrade.  I know that there can be a lot of variation but if someone was able to either give me a range of values it would be between or a link to this information it would be much appreciated.

By my own rough tests I have found it to be 382 cP - would this be a figure that could be possible for 15w-40 oil at room temperature.  The reason I ask is that I have been conducting an investigation into stokes law but I need to know approximately what the true viscosity of the 15w-40 oil I have been using is so as to make a fair comparison.
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### RE: Viscosity of 15w-40 at room temperature

It would be in the vicinity of 400-600 cS at 20oC.

Different makes have somewhat differing viscosities, mainly depending on the VI and the viscosity at 100oC.
These are generally expressed in centistokes (cS).

If you try a Google search on any particular brand asking for the MSDS you'll probably find the viscosities at 40 and 100oC, or even at three temperatures, the third one being a value for below-zero Celsius levels (the reason for the W).

An ASTM or BP viscosity-temperature chart would then enable you to interpolate -or extrapolate as the case may be- to the required temperature with a straight line.

### RE: Viscosity of 15w-40 at room temperature

If you obtain the viscosities at the reference temperatures at both 40degC and 100degC then you can use the ASTM D341 equation to find the viscosity at other temperatures.

This is actually a log realtionship:
log10.log10(ν+0.7)=A-Blog10(T+273)
where ν is the kinematic viscosity at T deg C.
A & B vary significantly with a change in quality so you need the viscosity at two temperatures to solve for A & B. Once you have these you can solve for the viscosity at any other temperature.

You can download a spreadsheet ASTMD341.xls from www.ViscoAnalyser.com or from www.cheresources.com (the link is in one of the newsletters) for this calculation and plot several oils at once. This equation corresponds well with proprietary equations such as Castrols Blend 42 and works for all lubes.

### RE: Viscosity of 15w-40 at room temperature

Geeaea,
a good calculation.
If you know the actual values, rather than the nominal values for the oils, try to use them... it can make a difference.

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