Oil Viscosity question....another one

HI again. quick oil question.
What do the weight numbers stand for?
i.e. 5W30.......................is the "5" the viscosity of the oil when fully hot?
or is the "30" the viscosity when fully hot.
There has been some debate about this. I've heard that multi viscosity oil has polymers in it that keep the weight down to a 5 so that cold startups have better flow. And as the oil heats, the polymers expand and bring the weight up to a 30. And I have heard that the 5 is the heated up weight, due to oil getting more viscous as it heats.

Anyone know the real truth behind this?

Without looking up the finer details, my understanding is basically this:

The lower number = lower viscosity. Teh "W" in the designation desribes viscosity at 0 degrees Fahrenheit. A multi weight oil should behave like the lower numbered viscosity when cold, and the higher number when hot. This can be misunderstood, if not careful. 5W-30 will behave when cold as straight 5W would when cold. It does not become as thin as hot 5W. When it's cold, it still pours like cold 5W, which might well be thicker than room-temperature 30. When it's hot, it pours like hot 30, which might well be thinner than room temperature 5.

The API/SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil. The first number associated with the W is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as cold as a single-grade 10 weight oil can be pumped. "5W" can be pumped at a lower temperature than "10W". "0W" can be pumped at a lower temperature than "5W", and thins less at temperatures above 99°C (210°F). The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. The governing SAE standard is called SAE J300. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle.