One property of most oils intended for use in internal combustion engines, is that it "thickens up" when the temperature drops.

This is an undesireable quality when temperatures start to drop near freezing. When you start an engine cold in cold weather, a thicker oil takes longer to circulate, and will deprive an engine of desperately needed lubrication, possibly causing metal to metal wear.

To combat this tendency in oil, viscosity index modifiers were developed to allow the oil to flow like an oil with a "thinner" rating in cold weather.

The "W" in an oil like 20W-50 means "winter". 20W is an indication of the low temperature performance of that oil. The lower the number that comes before the "W", the faster the engine will be able to crank and the faster the oil will flow through the system. It would be inaccurate to say that "20W" means the oil will flow like a 20 weight oil would flow at a given low temperature, but essentially that's sort of the idea. The number is actually an arbitrarily assigned grading scale resulting from a specific API defined test.

Some synthetic oil manufacturers say that they don't actually use viscosity index modifiers in their oil. Mobil 1, for example, claims that the formulation of their oil is such that it resists thickening when cold and thus flows like a conventional oil with a similar rating would that does use viscosity index modifiers. The implication of that statement can be interpreted to mean that they didn't deliberately set out to make a 15W-50, but it flowed so well under cold conditions that it's what the oil ended up as.

In their case, the 15W doesn't mean the oil is thinner at all temperatures, it just means that it flows well in cold weather. But since it's a sliding scale, it *is* thinner at ambient temperatures, just by it's nature. At operating temperature it should be the same as any 50 weight oil. Well that's how it's all supposed to work.