Many machines in our daily life require some form of lubrication, often found in the form of a liquid. The most obvious example would be your car's engine which requires somewhere between 4 and 6 quarts of oil to safely operate. This is example is a great learnin' tool, so let's rap.
Viscosity
The grade, or weight, of an oil is described by a number such as "90 weight". This is actually the oil's viscosity. Viscosity is measured as the amount of time it takes a certain amount of oil to pass through a certain sized orifice at a certain specific temperature. If you want to know those specific numbers, ask SAE. Low numbers like 10 indicate a fast moving and consequently thin oil. High numbers like 90 indicate a very slow moving, thick oil. Long story short: viscosity is quite literally how fast oil moves.
Now, have you ever handled an oil at different temperatures? Think about pouring oil in a cold skillet: how slowly does the oil roll around while you try to coat the bottom of the skillet? What about when you're done cooking and you can pour the oil out like water?
If you have ever changed the oil in your car when the engine was still warm, well. . . I don't have to explain that example at all.
The main idea is that oils become thinner and flow more easily as their temperature increases.
In your engine, your oil performs four vital functions. It lubricates, seals, cools and cleans your engine. And no, not that kind of cooling--there is a world outside of E2, remember?
It is beyond the scope of this node, but the oil travels through your engine via small passages that measured in the millimeters. If the oil is too thick, the engine cannot move the necessary volume of oil through the passages. Simultaneously, oil forms a protective shield between the pistons and the cylinder walls. That shield measures in the thousandths of an inch. If the oil became too thin and those surfaces came into contact, you could lose the entire engine. Assuming, of course, that you live through the wreck.
By adding polymers to regular oil, chemical engineers can produce an oil that has two effective weights depending on its temperature. Before the engine warms up, the polymers contract and float around without purpose. As the engine reaches operating temperature, the polymers expand to ensure that the oil's viscosity will not fall beyond the lower grade rating.
The worst routine engine wear occurs when you start your car. All the oil is in the oil pan, instead of being distributed throughout the engine. If we used a very thin oil, we could quickly pump our engine its lifeblood through those tiny passages. However, about the time we got to Kroger the oil would be too thin to separate the pistons from the cylinder walls and we have a seized piston. And we have to take all our groceries home on the bus. Double whammy!
The multigrade oil is the answer to our prayers. We get a thin oil for our startups and a medium weight for our actual driving. A 5W-30 grade functions as a 5 weight oil when it's "cold" (ambient air temperature) and a 30 weight when it's "hot" (at the engine operating temperature). Tada!
There you have it! Multigrade oil explained.
Bonus! While you are usually supposed to change your oil every 3 months or 3,000 miles, you actually don't. Oil doesn't wear out. What you actually have to replace are the vital additives in the oil like the polymers that produce the multiweight properties.
Double Bonus! Now, when you see those motor oil commercials on TV, you can get pissed off like I do! They claim that their oil is "heat activated" but now we know that all oils change in the presence of heat. They think we're all morons.