I found the article, though it is not free for download but can be read at

Stan Toepfer Understanding Motor Oil Viscosity - Documents

The quote most relevant to my statements is

"Advances in petrochemical engineering soon led to the development of chemical additives (viscosity enhancers) which when combined with a motor oil increased it’s viscosity index making it possible for a single oil to meet both the low temperature and the high temperature grade specifications. The molecules of these additives unwind as the temperature of the oil increases and slow down the rate at which the oil’s viscosity decreases. Additive enhanced oils which met both warm and cold temperature specifications became known as multi-grade oils. A major drawback to early multi-grade oils was oxidation of the viscosity enhancers during normal engine operation. As they oxidized they left harmful deposits inside the engine and their ability to maintain viscosity at higher temperatures degraded – simply put they were dirty and they wore out. Subsequent research and development has significantly reduced but not fully eliminated these problems. Development of fully synthetic motor oils has however produced multi-grade oils that do not incorporate viscosity enhancers.These oils are produced by breaking hydrocarbon molecules (typically ethylene gas) apart and recombining the component pieces into oils with entirely new molecular structures. These entirely man-made synthetic oils have extremely high viscosity indices. The downside to synthetic oils is their expense – they are expensive to produce and cost 3X or more as much as conventional oils. That expense is offset somewhat by the fact that they are very clean running and have extended life spans."

So, as I mentioned above, it is not an issue with every 20W-50 oil sitting in a crankcase; but it can and does still happen with the conventional oils. I had forgotten that truly full synthetics with no viscosity modifiers are not affected.

Growing up, devoid of any real automotive interests, I would always see engine oil ads on TV where all he manufacturers were trying to hawk the fact that THEIR oil was the best at withstanding (not preventing) "high temperature/thermal viscosity breakdown."

So THIS is what they were talking about............finally it has come full circle in my mind...

 

I know, taken as read.

And phenomenon's have been re-defined as time had brought out new information.

In my files (accumulated as a hobbyist) if have one entitled "Practical Lubrication for the Automobile" published by Standard Oil Company of NewYork 26 Broadway featuring the application of SOCONY petroleum Products.
It is a 60 page reference manual that discusses routine care and maintenance of your automobile at the time. Printed in 1923.
The discussions are pretty much in depth and on page 44 begins a 4 page directive on the proper weights of Socony Oils to use in the engines,(Winter and Summer) transmissions and differentials.
The weights of the oils are defines as
LM for light mdium
M for medium
H for heavy
EH for extra Heavy
G for Gear oil
Thus follows and alphabetical list of automobiles ranging from ACE to Winton followed by a listing of Trucks, tractors and boat motors.
One line might read (taken at random) for the Hudson for the year 1922 -1923 uses Medium grade oil in the summer and Light Medium oil in the winter. Where as the recommendation for a Mercer would read Heavy Grade oil for the summer and Medium grade oil for the Winter.
The point simply being a lighter viscosity oil in general was used during the winter and a heavier one during the summer.
A pattern held true to the time in about the 1970's when I was not to many years out of the service and stationed in the south where light grade oils simply were not carried as were things like anti-freeze. There was no need for anti-freeze, it did not get cold enough to use it and the good old boys in the south did not waste money on it. When the cooling system broke down, just fill it up with water.
A fairly good reason why for example, the aluminum engines introduced at the time universally failed. The infrastructure did not exist to supply them with the coolant needed to keep them from rotting out the coolant passages. I know I had that problem after replacing a blown gasket in a French car I owned at the time. As Anti-freeze was not available the radiator plugged solid in 6 months after making a repair on an engine that had an aluminum cylinder head.
Then it was also common to use 20W in the winter and 30 weight during the summer and straight weight, not detergents. Which brought about long discussions as to the benefits of both detergents and multi-weight oils as to the benefits of the extra costs involved in purchasing and using these new technology products.
Products now commonly used and the discussions "the same ones" are applied to the newest of the petroleum products.
Thanks for the reference to the article, now I gotta go read it.

 

Edited.
Derf
Trying to catch up with reading and taking care of things around the house has been a chore.
Synthetics probably have these characteristics, I don't really know which means I need to read some but putting a time line on things as I see it and I think is fair. Synthetics, while they were developed and used during WWII and maybe earlier, they were used by in large due to lack of having an oil industry for the use of lubrication during the war. My reading pretty much limits them in general to the German developments during that war. I rarely if any see reference to them as applied to other theaters.
As a teenager in the 50's and just learning to drive and take care of a car, even detergent straight weight oils had to develop a popularity due to increased costs of buying them
In addition, there were a number of models of automobiles that an OiL filter was still an extra cost option on the engine. Nash for one comes to mind and I think the I-6 Ford truck I once owned, a 1960 model the oil filter was an option for it. Either that or it was the spin on oil filter was an option. But air filters were either pretty much non-existent or they had strainers in them that kept large bugs out of the carburetor. Oil Filled filters took a while to be replaced by the paper air filter and piston ring wear was big time based in part on materials and in part on poor filtering of dust.
In 1960ish my parents came into the south where I was stationed and were not able to get a 10W30 Multi-weight oil for their car for an oil change prior to going back north during the winter.
Event the assumption that the advantage of multi-weight synthetics are universally used is not really true as at least in 2007 when I bought a New car synthetics were against the warranty requirements of the engine.
One can not assume the advantages if any are not universally used in that dealers are offering (at least the ones I have been at) optional synthetics on their oil changes.
And then as a cost benefit, and that too is real, once a warranty id off the car and in general that time interval is that of a 2nd owner, paying the costs of a synthetic oil vs a petroleum based oil is a function of the economics of the next owner. The dealer or manufacturer no longer holds any claim on warranty service at that point in time.
Granted, oil change interval of synthetics is anywhere from 3 to 5 times longer than a petroleum based oil with the exception of a routine filter change during the service life followed by replacing the oil lost by te filter change. This off setts the costs of the synthetics while not hurting it's performance. It makes using a synthetic oil a cost effective approach.
But not all people subscribe to that.
So many cars today are sold as an appliance rather than something that requires maintenance in order to supply reliable service.
This is of course supported by me anyway in the number of times I have gone on to a dealer lot in the last couple of years and been approached by the friendly salesman offering me a ride in their golf cart to go and choose a car OR they will bring just what I want up to the show room and let me look.
And the salesman in question at the time getting their panties in a knot when asked to see if the features I want are actually on the car they want to show me.
Yuh, things are changing and when technology is applied will, but even back in the day, Heavier oil weights were recommended for summer use vs lighter weights for winter and multi-viscosity oils might not be all that multi-in the long run. But in many cases, the recommended oil change interval was modified so that those variations as a function of time and use were not a significant factor in anything.