I live in a moderate climate. It is my simple and perhaps simplistic understanding that all cars use something called 30 weight oil for lubrication. But it is recommended that a lighter break-in oil, like a 10 weight is used for a period of maybe 10,000 miles on brand new engines so that all the bearing surfaces have time to adjust and wear in, like a new shoe. The 10 weight oil is intended to be a bit thin to allow it to happen. After this initial time it's all 30 weight from here on out. This makes sense to me.
However America has some places where it gets damn cold or damn hot. What happens to the oil? Well cold oil is thicker and hot oil is thinner. So these wonderful engineers at all the oil companies and car companies, who have a lot of time on their hands, have come up with the concept of multi-viscosity.
Multi-viscosity oil contains a polymer additive (about 10 to 15 percent by volume, so it obviously reduces the ability to lubricate) that consists of a weird hydrocarbon chain that is supposed to keep the oil thin at low temperatures and then magically makes the oil thicken at higher temperatures. The multi-vis polymer is a hydrocarbon chain that gets real long at lower temperatures then tries to get shorter with heat and thus imparts this viscosity to the entire lubricant. Not true as I will show. This is what the car companies and oil companies claim because they have their manufacturing resources set up to produce so much of this kind of oil and they expect the consumer to buy so much of this oil.
Lets take about 3 ounces of multi-vis oil, say 10w-30w and the same amount of straight 30w and put them in pans and heat them on a stove to 200 degrees, the normal operating temperature of a car. You will need a turkey thermometer for this test. Now pour both oils out onto a bowl. You will note that the multi-vis runs like syrup and the straight oil runs with notable viscosity.
Because of this I strongly avoid the use of these so-called multi-vis oils because they just do not work as advertised. Many people seem almost shocked when I say things like “I don’t Understand why anyone would use multi-vis oil in this area or why they even sell it here in Seattle?” or “Why use multi-vis anyway because it usually breaks down into regular oil after 1000 miles.” or “Regular oil lasts a lot longer so why bother with multi-vis oil.”
If you live in a moderate climate ignore the multi-vis oils. If you live in a hot climate then you have even more reason to avoid these things. If you live in Minnesota or Alaska during the winter, you probably don't want to drive anywhere anyway. But that's the only time I can see the practicality of using such oils.