Engine Oil Viscosity

Viscosity is the most misunderstood aspect of oil and yet it is the most important.

Viscosity is the force required to shear (break) the oil at a certain speed and temperature. Oils work because they have viscosity; the drag of a rotating part pulls oil from a low-pressure area into a high pressure area and “floats” the surfaces apart. This is called “hydrodynamic lubrication” and crankbearings depend on it.

Oil must be capable of flowing at low temperatures, so that it gets around the engine in a fraction of a second at start-up and must protect engine components at high temperatures without evaporating or carbonising and maintain adequate (not excessive) oil pressure. Many people think that the thicker the oil, the better the protection, but if the oil is too thick, it will not flow properly, leading to reduced protection.

The numbers on every can of oil indicate its performance characteristics when new but there are many misconceptions on what these numbers actually mean.

For multigrade oils you will see two numbers (for monograde oils only one). The first is followed by a “w” and is commonly 0, 5, 10, 15 or 20. The second number is always higher than the first and is commonly 20, 30, 40, 50 or 60. The first and second numbers ARE NOT related.

The “w” number (0, 5, 10, 15 or 20)
When multigrade oils first appeared, a low temperature test called “w” (meaning “winter” not weight) was introduced. Using a “Cold Crank Simulator", the test measures the oils ability to flow at low temperatures. ALL oils are THICKER at low temperatures than at high temperatures but the lower the “w” number, the quicker the oil will flow at low temperatures.

The second number (20, 30, 40, 50 or 60)
This number is known as the SAE (Society of Automotive Engineers) number and is measured in “Centistokes” (cst) at 100C. Centistokes (cst) is the measure of a fluid's resistance to flow (viscosity). It is calculated in terms of the time required for a standard quantity of fluid at a certain temperature to flow through a standard orifice. The higher the value, the thicker the oil.

An oils cst at 100C determines its SAE rating within the following parameters.
SAE 20 = 5.6 to less than 9.3cst
SAE 30 = 9.3 to less than 12.5cst
SAE 40 = 12.5 to less than 16.3cst
SAE 50 = 16.3 to less than 21.9cst
SAE 60 = 21.9 to less than 26.0cst

ALL oils labelled 40 must fall within the SAE parameters at 100C so everything from a monograde 40 to multigrade 0w-40, 5w-40, 10w-40, 15w-40 or 20w-40 are approximately the same thickness at 100C.

Some oil companies label oils as SAE 35, 45 or 55, but as you can see from the above figures, there isn't a SAE 35, 45 or 55. This "could" be because they are approximately on the boundary of the two grades, but as we don't deal with any of those I can't really comment further.

Summary

Cold start.
A 5w-40 will flow better than a 10w-40.
A 10w-50 will flow better than a 15w-50
A 5w-40 is the same as a 5w-30

At operating temperatures.
A 10w-50 is thicker than a 10w-40.
A 15w-50 is thicker than a 5w-40
A 0w-40 is the same as a 10w-40

If you look above, you will see that the figures quoted do not indicate at all as to whether the oil is synthetic or mineral based... Well except for 0w oils as synthetic PAO basestock is required to acheive this viscosity.

Generally the oil you use should be based on the manufacturers recommendation found in the owners manual, but then modifications, climate and the type of use can affect that recommendation. If you are unsure of what is the correct recommendation for your car and would like to know more please contact us here [email protected]

With thanks to John Rowland of Fuchs/Silkolene

Cheers

Tim and the Opie Oils team

The only "brand name" oil I'd swear to having had a viscosity not exactly divisible by 10 is Castrol R SAE55.

 

Since that's a vegetable oil and not a mineral base, I'd think it's "different" in that respect (and not recommended for anything except race engiens anyway).

The only "brand name" oil I'd swear to having had a viscosity not exactly divisible by 10 is Castrol R SAE55.

 

Since that's a vegetable oil and not a mineral base, I'd think it's "different" in that respect (and not recommended for anything except race engiens anyway).

This oil also smells very nice !

hi,what's the normal engine oil temperature should be on Skoda superb 2.0 ? Thank you

So, can a Skoda engine monitor it's own engine oil viscosity to help determine the long life (variable) service intervals?

 

Something I've always been curious to understand, some say yes, others say no.

So, can a Skoda engine monitor it's own engine oil viscosity to help determine the long life (variable) service intervals?

 

Something I've always been curious to understand, some say yes, others say no.

I'm not sure the sensor measures the viscosity directly (that's a measure of thickness, and usually measured by the time it takes a known quantity to drain through a calibrated orifice) but other qualities of an oil vary with age too.

It could be done, with the existing oil level and temperature sensors, by recording the rate of level drop at engine start-up, as the oil pump starts chucking oil up the engine.  If that rate changed systematically over the course of a service interval, after correcting out temperature differences, you might be able to detect thinning (or thickening) of the oil?  How it'd decide 'right that's far enough, time for some fresh oil' is beyond me though.

 

Actually, having just looked at a log I made of oil level on a short journey recently, that wouldn't work at all.  The level barely drops at all at start-up (at about 10 seconds on the graph, see rpm trace).  Oil pressure relief valve will be a complicating factor, with cold oil much of what is drawn into the pump will be chucked straight back.

 

Could better be done at shutdown, measuring the rate of level increase as oil returns to the sump. Except the sensor and cluster that does the signal processing would be powered down then, I think.

Edited 22 August, 20169 yr by Wino