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I'm glad someone posted a chart on this it doesn't tell the complete story but it is good enough for a reference

According to the standard handbook of mechanical engineering the SAE viscosity rating with the W is based on SUS standard. Others without the W is not. The viscosity viscosity is based on to temperatures 0 degrees Fahrenheit and 210 degrees Fahrenheit. a maximum and minimum viscosity for each temperature is used. It is a very loosely applied standard which may include a wide range of viscosity. A chart given in the book shows SAE rated oils vs. the SUS standard. At approximately 120 degrees SAE oils x10 follow the same viscosity as the SUS rating. In another section of the book describes converting SAE rated as follows ( W x 10 ) it is only a rough conversion not intended to calculate actual viscosity the actual viscosity can only be determined by a testing facility

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Rick