Tiny,
I guess my credible source would be the Society of Automotive Engineers.
Longbox was 'spot on' as they say.
A multigrade oil is formulated in such a manner that it will exhibit different viscosity's at different temperatures.
As Bill noted, the 'W'(WINTER) after the first number of a multigrade indicates its measured viscosity at ZERO degrees.
The second number is the measured viscosity at 210 degrees Fahrenheit.
A single grade oil will not have a 'W' behind it. This means the label viscosity is at 210 degrees.
This concept doesn't mean the oil gets thicker with a rise in temperature, it means with the addition of VI (viscosity index) improvers that it doesn't 'thin out' at elevated temps.
If you 'Google it' you will find a wealth of info on the subject.
I am not a engineer, so some of my statements might not be accurate, but generally , this is how it works.
Stuart
