I believe the word ‘water’ is being used as a tongue-in- cheek reference to the low-viscosity oils used in many modern vehicle engines.
When draining 5W30 @ 150 degrees it flows faster than water. Why should engine oil be thicker than water? Water is not a baseline for oil standards.
Ive been changing 0-20 oil for ten years and never noticed any iissues. Gotta remember though zero is the cold weight. Once heated, its 20. Spend dome time with the oil geek though.
I’m so old…
I’m so old my first two cars required 10w40 oil! 
I think I’m in good company on this site when I say my first car specified
- SAE 20 or 20-W in normal summer weather,
- SAE 30 above 90 degrees F or high speeds,
- and SAE 10-W for winter.
In Wisconsin in January my Dad would run 5wt non-detergent in our Flathead six. My first generation Coyote is out of warranty, next oil change it may get 0w-30 0r 5-w30 full synthetic rather than the 5w-20 blend.
As far as everybody else, if the car is out of warranty run what you want. As far as I’m concerned you can run Mazola or Karo syrup.
Good to see a robust verification plan has been executed. 
When the designers decided to use the thinner weight oils to increase efficiency, it wasn’t done in exclusion to everything else. That system is designed as a whole to use that specified oil. In fact, to take advantage of it in other ways.
For example, the design interface tolerances can be made much tighter as a result. A thicker oil will take longer to properly access and lubricate those interfaces resulting in increased wear.
The thinner oil is transferring heat more efficiently and the engine design takes this into account. Using a thicker oil risks overheating critical areas. This will not be immediately obvious. For example, increased wear on the rings, turbocharger bearings, breakdown of the oil itself…
Many modern designs use the oil as a hydraulic control fluid and thicker oil prevents these systems from working as designed until the oil heats up enough.
The list goes on. Trying to outsmart the people that designed it usually doesn’t end well in the long term…
I don’t believe any oil becomes thicler (heavier) as it gets warmer.
That is what the multi weight means as opposed to a straight weight, but ill check again.
You can do a quick goggle for an AI answer plus a video but excuse the english accent. A lot of that going around. If you want an engineers answer then see the oilgeek.
All oils get thinner as they get warmer. Multi-grade oils (take 0w-20) act like 0 grade at cold temperature (1 mm2/s at -40C) and like a 20 grade when hot (9 mm2/s at 100C). But it’s still far thinner when hot than when cold.
Well duh, but it would be the same viscosity with a straight 20wt as a multi weight a engine temp.
Now to complicate a little, the oilgeek talks about the ability for the oil to stay in grade. So you may think yo have a 30 weight but it actually degraded to a 10.
I put 5W20 in my 1998 Dodge Ram and everything was OK for 3000 miles, the engine was quiet, the sky was blue… No different than 10W30.
You were probably using the wrong viscosity oil, was this a vehicle from the 1960’s?
'71 Chevelle in high school,
'81 Buick Century in college
Wouldn’t a 0w30 have the same amount of viscosity. Modifiers?
Well, that’s what “My father said” to use, and that was in 1990, around the time I was driving the '81 Century. My Google search results match your statements about 10W30.
Drove it for six years retatively trouble-free, being it was an estate car with under 5,000m(8,000km) on the clock. Oil, two sets of tires, one A/C recharge, one set of sparkplugs, is all.
My 2017 Chevy Volt requires Full Synthetic 5-20W “Dexos” oil. It is stated on the oil cap.
I went to my local Wally-World to buy some Mobil 1 Dexos 5-20 and I found it but the Dexos label was gone. They had 0-20 and 0-30 with the Dexos label. Checking online it appears that Mobil has to pay GM for certification to place “Dexos” on their oil. Further searching showed Chevy saying it was “ok” to use 0-20 in my Volt. But my cap says 5-20.
So off to my Volt messageboards/forums. Some guys were saying “It’s ok to use the thinner stuff”, others thought that just because Mobil didn’t pay the blood money to GM didn’t mean the 5-20 was any worse for my car than before, when it did have the label! What did I do? I bought the Mobil 1 5-20W (without the label) and have used it for years now.
“Dexos” is marketing.
5W20 is the spec oil. Use that.
One n done.
I disagree. 5W20 is a viscosity rating. Dexos is a durability and performance rating. Vastly different. The Dexos rating is required to ensure proper operation of the oil life monitor. Using the manufacturer recommended oil change interval without using the proper oil could lead to problems. Why take the chance?