yeah flow is important too. But you only need so much, you need a balance in the end. Plus you should know as well as anyone, engines change over time. Clearances get smaller parts get smaller. So you do need an oil that will deform into smaller areas. I think though one part you are still missing is that under almost all circumstances, oil molecules will fit between moving parts, unless there is to much heat build up. But thats not my point... you measure oil molecules in terms of microns. A micron is a Millionth of a meter.
1 micron is equal to 0.0000393701 inches. Now an oil molecule is ~0.001 microns. so that means 0.0000000394" so... Pretty small. That means that you can fit a lot of oil molecules between parts. So in this instance we are concerned with viscosity in relation to surface tension, or how easy it is to break that tension to deform it to fit.
Now once again, like I have said before, this surface tension is great for keeping oil inside a seal. This surface tension makes the oil not flow back as easily through the clearances around your oil pump and between the impellers. Reducing the surface tension to the point where it deforms easier, means that oil can flow against the direction it is being pushed and through seals.
Here is where I get back to my original point about thermal expansion. If all the parts have shrunk, and the oil molecules have shrunk, what keeps the oil from flowing through cracks it is not intended to. Its viscosity, or resistance to deform enough to freely move through the very small cracks. This is why, despite the fact oil "thickens" when it gets cold, that isnt a bad thing, because all the parts shrink, which means all the gaps grow.
Now what you pointed out earlier, is that lower weight oil flows easier, your engine is going to move a constant volume of oil no matter what the weight of the oil. Lowering the resistance to flow to a point reduces parasitic drag on the engine. You can see how with a lot of engines companies sell high volume oil pumps. They are called high volume because the oil pump moves more oil.
Regardless of weight, an oil pump moves a specific volume of oil. This volume is constant, regardless of viscosity and temperature. The viscosity of the oil as it meets resistance to that flow is what creates oil pressure and forces the oil to go into smaller areas. If you did not have this resistance, the oil would just flow over the small areas right back into the pan, you could circulate the oil around the moving parts without any of it flowing into the moving parts since the force exerted between moving parts would resist the oils movement into the gaps between those parts. So imagine if all of your oil flowed from the pan, onto the top of the heads, then trickled back down.
When the engineers design the motors, they determine what minimum and maximum tolerances there will be in an engine based on what conditions it is likely an engine will meet. So an engine designer, building an engine wants to determine what conditions an engine will see if it is in a passenger car in the US. So if you determine you need to build your engine to start normally between a temperature of -15f and 120f, figuring that an engine probably wont get much colder than that and be able to start, since in most cases you are gonna need a block heater to start your engine if it is even that cold, so your engine will not likely be any colder than that. So now they have a set of maximum and minimum tolerances for starting, they can set tolerances for operation, figuring their engine is going to be cooled by the coolant system, that means operation will be between startup temperature, and operating temperature of the engine, which should be ~210 at most. So they take these parameters and determine from there that they are the extremes of the operating conditions.
You take those extremes, add a margin for error or abnormal operation, such as cooling failure, or cold beyond what they would expect their engine would be able to start operating at. From there you can take your calculations and determine what the clearances will need to be between moving parts based on the thermal expansive properties of the materials. A large part of this determination is fluid viscosity at specific temperatures. So... Part of designing an engine is knowing before you design it what your oil will fit into, so in a way, you design an engine around the oil you intend to use in the engine. From that you could infer that you dont need a lower or higher viscosity unless your engine is operating outside of the parameters the engineers intended it to, or if the parts of your engine is not withing normal operational specifications.
Long post, sorry to anyone who read the whole thing.
. Although I am sure if I search long enough I can find engineering papers on this already, I will let you know if I have any luck.