Oil Viscosity

In a 5W/30 oil for example, I understand that the W after the first digit stands for "winter" or "cold" level while the next two digits stands for the viscosity after which the oil lubricating performance breaks down. My question is: At what temperature level has the engine to operate before it reaches the oil brakdown point. My point here is that by the time the engine reaches a temperature level which would cause the oil of that weight to breakdown, many parts of the cooling system such as the radiator, hoses, water pump, etc. would have already broken down correct?

I always thought the 5W was the weight of oil in the cold and the 30 was the weight when it was warmer. Obviousy there are specific temperatures but winter/summer gets the point across.
Oil will breakdown/oxidize eventually at any temperature but the hotter it gets the shorter the lifespan....Mike

5w/30 means the viscosity weight of the oil at cold engine temp is 5 and as the engine hits warmed up temperature around 200 degrees it wont thin out more than 30 viscosity. Viscosity is measured by how long it takes a liquid to flow through a certain size hole, kind of like sand in an hourglass. I don't believe vicosity has anything to do with the oil actually breaking down. just how thin it gets at different temperatures

Check out http://www.carbibles.com/viscosity/html.