5W30 - 10W30 - 15W40 Engine oil

What's with these numbers? 5W30 - 10W30 - 15W40 As I read all the post about different weights of engine oil. How many people know what these numbers stand for. These are Multi-grade oils. The temperature range the oil is exposed to in most engines can be wide, ranging from cold temps in the winter before the engine is started up, to hot operating temps when the engine is fully warmed up in hot summer weather. A specific oil will have high viscosity when cold and a lower viscosity at the engines operating temperature. The difference in viscosities for most single grade oil is too large between these extreames of temperature. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers are added to the oils. These additives are used to make the oil a multi-grade engine oil. The idea is to cause the multi-grade oil to have the viscosity of the base grade when cold and the viscosity of the second grade when hot. This enables one type of oil to be used all year. In fact, when multi-grades were initially developed, they were frequently described as all-season oils.

The SAE designation for multi-grade oils includes two viscosity grades: for example, 10W30 designates a commom multi-grade oil. The first number "10" is the viscosity of the oil when at cold temperatures "zero (0) degrees" and the second number is the viscosity at 212 degrees.
So it's like this "A 15W40 pours like a 15 oil when cold and lubricates like a 40 oil when hot". Thanks to viscosity index improvers.

I hope this helps some of the oil confusion, Sorry, this got a little long, but it just had to be explained.
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Maybe I was taught wrong in my technical training, but I was under the impression that say a 10w30 has the viscosity of a 10, but the load carrying of a 30. Your link did not load for me.

Oh Boy here we go an oil post. This will get the experts going.

Due to the fact that several posters ask about oil recommendations, I see no problem with Charlieu's very good explanation of multi grade oils. Like he said, the polymer's are what keep the oil from thinning out at higher temperatures and just to throw this in, they are the reason IH and others used to recommend straight weight oil, to stay away from polymers. Their reason, not mine and they have all changed their minds now days.