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Thread: What does 10w-40 mean..?

  
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    What does 10w-40 mean..?

    10w-40, what does it mean?
    We have answered thousands of oil questions in the last 4 years but the most frequently asked one is "What do the numbers mean?"
    In short, if you see an expression such as 10W-40, the oil is a multigrade which simply means that the oil falls into 2 viscosity grades, in this case 10W and 40.
    This is made possible by the inclusion of a polymer, a component which slows down the rate of thinning as the oil warms up and slows down the rate of thickening as the oil cools down.
    Multigrades were first developed some 50 years ago to avoid the routine of using a thinner oil in winter and a thicker oil in summer.
    For a 10w-40 to attain the specification target a 10W (W = Winter please note!) the oil must have a certain maximum viscosity at low temperature. The actual viscosity and the temperature vary with the viscosity grade but in all cases the lower the number, the thinner the oil. For example a 5W oil is thinner than a 10W oil at temperatures encountered in UK winter conditions.
    This is important because a thinner oil will circulate faster on cold start, affording better engine protection and therefore lower long term wear!
    For a 10w-40 to attain the other specification target a "40" oil must fall within certain limits at 100 degC. In this case the temperature target does not vary with the viscosity grade, if there is no "W" the measuring temperature is always 100degC.
    Again the lower the number the thinner the oil, a "30" oil is thinner than a "40" oil at 100 degC, which is typical of maximum bulk oil temperatures in an operating engine.
    Engine makers are, of course, very well aware of this and specify oils according to engine design features, oil pump capacities, manufacturing tolerances, ambient temperature conditions etc. It is important to follow these guidelines, they are important and are stipulated for good reasons.
    Finally, if the engine has been modified or is used in stressed conditions, the operating conditions may well be outside the original design envelope. The stress on the oil caused by increased maximum revs, power output and temperature may require that an oil of a different type and viscosity grade would be required.
    These examples show viscosities at different temperatures:
    Grade................0degC............10degC...... .........40degC...............100degC
    0w-40...............665cst.............354cst........ ..........82cst................14cst
    5w-40...............842cst............ 430cst..................91cst................14cst
    10w-40.............874cst.............440cst .................91cst................14cst
    15w-40...........1260cst.............595cst........... ......107cst................14cst
    In a nutshell, that’s what a multigrade is all about!

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    Guy.
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    wow excellent, ive always wondered what all the numbers meant, thanks for that

    I use 5w 40 at the moment so from reading your post thats better for this weather we are having?
    400+ bhp Astra SRI Turbo
    mpg

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    So whats the difference between mineral, part- synthetic and fully synthetic. I was getting some new oil the other day and the car needed either 15-40 or 10-40. The women at the counter told me 10-40 was better for it because the temperature can go lower? Is that BS isit just what is needed?

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    Quote Originally Posted by futur3 View Post
    So whats the difference between mineral, part- synthetic and fully synthetic. I was getting some new oil the other day and the car needed either 15-40 or 10-40. The women at the counter told me 10-40 was better for it because the temperature can go lower? Is that BS isit just what is needed?
    If its to go in ur Astra then get 10-40 = END OF

    15-40 is just too thick for modern engines.... I use it in mine but thats cos its an old 8v design and I have 25Ltrs of the stuff but for your 16v unit stick with the recommended 10w40

    semi/synth/mineral - some say its marketing hype - some say its blah blah blah.... but as long as the oil meets the ACEA specs then youre sorted

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    Quote Originally Posted by davy View Post
    wow excellent, ive always wondered what all the numbers meant, thanks for that

    I use 5w 40 at the moment so from reading your post thats better for this weather we are having?
    Yep, 5w-40 is ideal.
    Use the code ASTRAOC and get 10% Club Discount
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    Quote Originally Posted by futur3 View Post
    So whats the difference between mineral, part- synthetic and fully synthetic. I was getting some new oil the other day and the car needed either 15-40 or 10-40. The women at the counter told me 10-40 was better for it because the temperature can go lower? Is that BS isit just what is needed?
    The difference is what they are made from. Mineral oils can only span a small viscosity gap, ie 15w-40, semi synthetics be better quality can span a larger gap, say 10w-40. Synthetics being the most stable of the lot are able to span even larger viscosity gaps such as 5w-40 or 0w-40 ans still be stable where as a mineral oil will just fall apart.

    The way to look at it is 10w-40 semi is a cost option, 5w-40 synthetic is a quality option.

    Cheers

    Guy.
    Use the code ASTRAOC and get 10% Club Discount
    oilmans website : www.opieoils.co.uk/
    e-mail : oilman@opieoils.co.uk

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