Long time reader, first time poster here. I thought I would clear up a few things about oil. First of all, most people think the "W" in oil stands for "Weight", but it actually stands for "Winter". So, a 15W40 oil indicates a 15W winter weight, and a 40 operational weight. These two weight "types" have different viscosity properties, so a 15W weight oil does not have the same viscosity vs temperature as a 15 weight oil.
So, the following is typically true about oil viscosity: 0W<5W<10W<15W @ 0C. I say typically, because the second number can impact the viscosity of the first.

If the second number is the same, then this statement is always true.
As for the second number, the following is always true about the viscosity: 20<30<40<50. So at operational temperatures, the higher the number, the higher the viscosity.
In the case of a 5W40 or 15W40 oil, the cold starting (0C) viscosity of 5W or 15W weight is always HIGHER than the operating temperature of 40 weight oil at operational temperature. So the oil viscosity for 10W40 is as follows: 10W @ 0C > 40 @ 100C.
So as far as seals, and part tolerances, it is important to make sure the second number always meets manufacture's recommendations. This ensures the oil isn't too thin at operational temperatures, which could push through seals of moving parts with poor tolerances and seals.
As for the first number, you can typically go lower, especially in colder states. The only thing to make sure of is that the "Winter Weight" viscosity isn't higher than the running weight, which I've never seen. Just keep in mind that the two numbers really have nothing in common. 15W weight does not equal 15 weight oil. It simply means WINTER Weight vs Operation Temperature Weight. Operational viscosity typically flattens out over 100C.
Here are some oil viscosity charts to digest, and give you a better visual of what I'm trying portray.
Engine oil ? viscosity table and viscosity chart