Viscosity Is Relative, Not Static...
First, read your owner's manual as regards recommendations for viscosity. The viscosity of a multigrade oil is determined by the climate you operate in, not just the year and model of car. For instance, I live in California, and for my cars, I need to use a higher viscosity rated oil, like 15W50. If I lived in a cooler climate, I could use 10W30, or even a 0W40. The "W" stands for "Winter", not "Weight"...
Some extremists believe that a 0W40 will work better for them even in California, because they think that the oil is at 0 viscosity when the car is started. That would only be true if the ambient temperature were cold enough to cause the oil to flow at 0 viscosity. If the temperature were say, 56°F, wouldn't the oil actually be about 10, or 15 anyway due to the rise in temperature from where the oil would be flowing at 0 viscosity? I don't know that answer yet, but I'd like to, so I will try to find out just what the temperate spread is on a multigrade oil.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."