Oil viscosity - why the 20W-50 BS?

Here is my 2 cents for what it is worth. Multigrade oils are oils that have viscosity modifiers in them. When oils are graded they must pass a flow test. The graded is determined by that test. A 20W-50 oil flows the same as viscosity grade 20 oil when cold. As the temperature of the oil increases the oil flows like grade 50 oil. Multi viscosity oils were developed to compensate for the extreme difference in oil flow rates from cold starts to normal operating temp. The issue arises in the fact the the viscosity modifiers do not last as long as the oil. This results in that 20W-50 oil flowing like a standard 50 weight at cold temperatures.
Another issue is that it takes energy to move the oil through the system. This is why you want to select an engine oil that falls within the normal temperature extremes of the environment you are driving in. If the weather is continually below zero you need a thinner oil. If you live in an area where the temperature never drops below 60 you do not need a 20W oil. That is why there are suggested temperature ranges for oil use. You want to use an oil that is thick enough to force the moving metal parts away from each other but not so thick that your engine has to work to move it through the system.
~Michael