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@MikeInNH and @Scrapyard_John - as they used to say, itâs just car talk.
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The article you provided on underhoodservice.com is quite diluted for the âregular folksâ (and still makes a mess), but even there it refers to the âbefore-Wâ number as a âpour pointâ, not as a âviscosityâ (and God bless not âzero viscosityâ):
It may be somehow related to the âReading Comprehensionâ discussed above, but we should be clear on terms.
Now, âpour pointâ measure is a threshold, not the actual value, which is nicely illustrated in this quite old study: https://dextrous-lubricant.com/amsoil/PDF/g3115.pdf
Here is a great comparison on page 6:
So, as you see, all oil under test were 5W30, so supposedly have a similar âpour pointâ (which sometimes happens to be misinterpreted as âcold viscosityâ), but in reality, the actual viscosity at -22F / -30C is very different between brands.
Moreover, this viscosity is much-much higher than hot viscosity of âafter-Wâ number or actual SAE rating.
Here is the sample spec for Valvoline synthetics:
The second line gives âcoldâ viscosity", where the the first one is for âhotâ.
Pay attention that one is at 40C, another one is at 100C, so âcoldâ is defined as âcar was sitting long on the July sunâ, not âcar was left overnight in Anchorageâ
For the âreally cold startsâ, that viscosity will be even higher⌠to the point where it does not flow anymore (but becomes a jelly), and this is where the definition of âpouring pointâ comes into the view.
Iâve specifically marked â20â and â30â as two âfamiliesâ here, where when âhotâ 0W20 and 5W20 (and 5W30 + 10W30) are functionally equivalent when hot, but clearly different between xxW20 and xxW30 groups.
So, I will repeat my point from above: when you talk about hot operating conditions, the only thing what matters is following the manufacturer-recommended âafter-Wâ (SAE) number, then you can pick and choose the âbefore-Wâ as it suits your environment.
Following the manufacturer spec is the easiest and the safest approach, there are many reasons to do so, but if one guesses what can be safely altered (or even used to the advantage over manufacturer recommendation), it is not a rocket science to decipher the oil grade index.
Got a ways to go to get to 400 posts, seems to be heading that direction.
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I could not resist not to make a final point on why I would (and I do!) use 5W20 in place of 0W20, same as I used 10W30 in place of 5W30 for years and years.
Look at Noack factor for 0W20 and 5W20: it is quite a startling difference in evaporation!
Same for 10W30 vs 5W30.
I live in moderate Virginia climate, so I do not need the extremely-low pour point, but I do need the least evaporation at high temperatures. For Florida it would make even more sense 
My old Subaru was burning oil where I needed to add in 5000 miles change interval for 5W30, but I did not need to add it for 10W30.
Considering that oil goes into the catalyst, thatâs yet another reason to go with your environment needs, not a âholly grailâ recommended by the manufacturer for âall continental USâ
All this discussion about substituting one viscosity for another is fine and dandy, particularly if your ownerâs manual has an Ambient Temperature chart that shows more than one viscosity, but the ownerâs manual for OPâs car does not show more than one acceptable viscosity; it shows one, and only one, viscosity: 0W-20. It doesnât say that 5W-20 is acceptable, and it doesnât say that 0W-30 is acceptable.
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Perhaps you can correct the statistic, but my recollection is that 90% of engine wear is within the first 90 seconds of operation. I use a block heater, but would not 0W flow through the engine faster than 5W?
The manual says 0W20, itâs a settled/hard fact, Iâm not contesting it.
Can I sue something else?
Yes, you can if you apply some knowledge - thatâs an opinion.
Everybody is entitled to have an opinion, as long as it does not go against the hard facts.
I made my point that under certain set of conditions/limitations, âequivalent or betterâ conclusion can be made.
The one may agree or disagree, Iâve provided my opinion and the justifying facts, it is up to individual reader to interpret, agree or disagree
The answer is âit REALLY dependsâ, as Quaker State Ultimate is much-much better flow than Red Line HP even for 5W, so most likely a combination of two brands/products can be found where brand A 0W will flow worse than brand B 5W
I need to find someplace to go in the universe to get away from disputes about what type and brand of oil to use. Itâs as bad if not worse among French horn players as auto owners.
One horn player told me last night at a rehearsal that she uses a mixture of Three-in-One oil and kerosene. I attended a horn clinic with a session on horn maintenance where the presenter was specific about not mixing synthetic valve oil with regular valve oil. I bought two bottles of synthetic rotor valve oil. One was a heavier oil and one was a lighter oil. Both bottles said that the contents were compatible with regular rotor oil. To add to the confusion, Conn instruments sells French horn rotor oil under the Conn label and Holton sells rotor oil under the Holton label much like Honda and Toyota each have their own labels on motor oil.
Taken from article above - The Society of Automotive Engineers (SAE) has established a numerical code system for grading motor oils according to their viscosity characteristics. Because the viscosity of oil changes with temperature, multigrade oils were developed to provide protection across a range of temperatures. This is why you would see something like this on the label: SAE 5W-30.
In a 5W-30 for example, the number before the W describes the viscosity of the oil at low temperatures. The lower the number, the thinner the oil and the better the oilâs cold temperature/ cold start performance. The number after the W describes how thick the oil is at the engineâs normal operating temperature.
Youâre saying the 0w is NOT viscosity, but pour rate.
Definition of Viscosity - Engine oil viscosity refers to how easily oil pours at a specified temperature. Thin oils have lower viscosity and pour more easily at low temperatures than thicker oils that have a higher viscosity. Thin oils reduce friction in engines and help engines start quickly during cold weather. Thick oils are better at maintaining film strength and oil pressure at high temperatures and loads.
Youâre confusing Viscosity Index with Viscosity Grade.
My 09 Focus only lists 5W-20, as does my 2005 Odyssey. My 05 Town & Country did as well. I donât think itâs as uncommon as it used to be
Seems? As Shakespeare said, âI know not seems.â
In that chart, and text, Honda is saying âWe specify 0W-20 for temperatures from lower than -20 degrees F to higher than 100 degrees F. Use a Genuine Honda Motor Oil or another commercial engine oil of suitable viscosity for the ambient temperature as shown.â
No mention (no recommendation) that it needs to be synthetic or not synthetic.
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You may use synthetic. It is not necessary. "Use a Genuine Honda Motor Oil or another commercial engine oil of suitable viscosity for the ambient temperature as shown "
Advanced reading comprehension skills are not needed here. Just understand the info in the manual.
A Honda dealer service department employee once told me that Honda recommended against synthetic oil. (I had been using it in my 1999 Honda.) I waited for him to say more and he said âIt doesnât have enough⌠grit.â Of course that was ridiculous. So is saying the Honda manual cited specifies that synthetic oil is necessary.
Advanced reading comprehension skills arenât needed to know what âno mentionâ means either, yet that was your claim to which I was responding.
And donât get me started on the "right"ways to adjust clarinet reeds. Or reed strength. Or, to be a little less off-topic, what, if any, oil should be used to oil the bore.
@Za1. I use Al Cass Fast down the valves, rotor oil on the upper bearings under the valve caps and I put sewing machine oil in a point oiler and do the lower valve bearings.
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I donât bother to adjust my reeds (Iâm pretty much a clarinet newbie) and I oil the bore with sweet almond oil. Musicians are nuts.
Ok, you made me to correct my understanding of the actual benchmark for the first number 
Now, for the point of âcan 5W20 used in place of 0W20â, it still not making much difference, as unless we go into low-temp extremes, 5W20 can be used in place of 0W20 with other things equal, and even has some advantage in the high-temp/high-load situations.
BTW, I officially claim post 100 in this thread