Originally Posted by ChrisSlicks

No problem.

Yes, it's not the viscosity "value" just the SAE rating index. The oil must exceed a certain viscosity index in order to qualify for a specific SAE rating. The lower the rating, the thiner the oil.

The split ratings such as 5W-30 simply imply that at 0F the oil behaves like a straight SAE 5 oil, and at 210F behaves like a straight SAE 30 oil. SAE ratings are always at 210F so yes I can see where what I wrote above might confuse some. Despite how much "stretch" the oil has it will always get thicker as it gets cooler, it's just a matter of how much.

The other thing to consider is to generate a large stretch such as 0W-40, many different base oils are mixed together to give it this characteristic. At very high temperatures the base oils used to generate the low temperature characteristics can break down, degrading the oil's performance and lowering its flash point.