You asked for a scientist, that's why everyone shut up.
Either that, or they don't have time to be a tudor.
I've been looking at this thread to get some good ideas on what to put in both of my rides as soon as the break-in is over. The popular choice seems to be Amsoil. But it's not as easy as driving to the local automotive store. But actually I was able to become a six month preferred buyer which allows you to buy it at "wholesale" prices. It's 20% savings, it's better than a sharp stick in the eye. And I can order it online and not even have to drive to the store.
As far as your question, do you really want a "scientific" answer? Because it doesn't have to be that scientific. The multi-viscocity oils will have a range of operation that they will be within the viscous. In otherwords, a 10W-40 will be thin enough for a cold start, yet be thick enough for a warm engine. A 0W-20 is going to thinner at even colder weather and thinner than the 40 weight when warm. It's not uncommon for manufactures to give multiple range options depending on the temperature that the vehicle is operating in. That's why you may need to use a lower viscocity oil during the winter and higher for the summer. Engine component tolerences and other things also effect the recommened oil viscocity. My Jeep wants a 0W-20 all the time because of tighter tolerences and it saves some on gas (for EPA rating). While my KTM wants 20W-50 because it's expected to run very hard in the heat of the summer.
Check you owners manual and it should tell you what weight you should run during that season. For example, you might run the 0W-30 when you expect to start it at or just below freezing and yet will not be running it when it's 98 degrees. And you would run the 10W-40 when you will NOT start it around freezing and you expect the run it when it is 98 degrees. Your manual should tell you what the ranges are for each oil. There is a lot of leeway here on what oil you run at what temp, that's why it's not really rocket science.
And if you would really like something scientific, I copy/pasted this from a quick search:
Viscosity is ordinarily expressed in terms of the time required for a stÂandard quantity of the fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid. Since viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. With petroleum oils, viscosity is now commonly reported in centistokes (cSt), measured at either 40°C or 100 °C (ASTM Method D445 - Kinematic Viscosity).
At cold temperatures, the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C, the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.
)
