Engine Oil Viscosity

  • 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.


    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 oilman@opieoils.co.uk

    With thanks to John Rowland of Fuchs/Silkolene


    Tim and the Opie Oils team


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