As there has been some discussion lately on oil viscosities I thought it might be appropriate to create a new topic heading.
It may not be known to many the true viscosity of your oil at different temperatures so I have included a link to a conversion calculator.
http://www.jiskoot.com/Calculations/Viscosity_temp/Viscosity_temp.html The universal world standard is to test viscosity is at 40 degrees Celsius and 100 degrees Celsius. The manufacturer will usually also give a cold cranking viscosity measured at a low temp depending on the start number (example 0w, 5w, 10w, or 20w) You must go into the manufactures technical specifications to get your true viscosity values. What you see on the label (example: 10w-30)is just a rough guide for use and not what the viscosity is most of the time. True viscosity will vary between manufactures and even between different products from the same manufacturer and only the technical information will tell you exactly what you have.
100 Celsius would probably be close to engine component temperatures, but that would vary with different parts, for example exhaust valve stems would most likely be much higher than 100C
As an example I've used an oil mentioned in a different post(Royal Purple) Their 0w-40 has a viscosity at 40C of 79.9, at 100C of 14.3 and a cold cranking viscosity of 20 000 at -40C. If you use the calculator, and you can use Fahrenheit as well, it will indicate a viscosity of 1416 at 10F and 2206 at 0F. Their 10w-30 at 40C is 65.27, at 100C is 10.75, at 10F is 1569, and at 0F is 2602, cold cranking viscosity is 12 000 at -30C, it would be 52 000 if rated at -40C.
By the numbers show 0w-40 has a more stable viscosity than 10w-30, that is it does not get as thick under cold conditions and it does not thin out as much as it gets hotter. This stability is represented by a number know as the viscosity index, the higher the number equates to a more stable viscosity. As a general rule the further the two numbers are apart on the package the higher the viscosity index. In comparing these two oils, 0w-40 has a VI of 182 compared to there 10w-30 which is much lower at 155.
If you compare their 10w-40 to the 0w-40 you would on face value expect it to be thicker at the lower temps and the same at the higher temperature. The 10w-40 is thicker as it gets colder but ends up thinner as well at the 100C when you might expect it to be the same and those differences become greater as it gets hotter or colder due to the lower VI of 162.
The different additives will also determine which oil is best for the intended purpose but a more stable viscosity is a good starting point and the higher the number the better.
So to reiterate, what is on the package is only a rough guide and to really know what your oil is doing and compare to other oils you have to look at the technical specifications because they are not all the same even if they have the same number on the package.