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Synskeptic: SynLube has this to say in their discussion of viscosity (very useful for a basic understanding of the subject):
Even more important is the High-Shear High-Temperature MINIMUM specification in SAE J300. In [the] tables below you will notice that there are "two" SAE 40 specifications, one with [a] minimum HSHT value of 2.9 cP for automotive Oils (SAE 0W-40; 5W-40; 10W-40) and the other for Heavy Duty Oils (HDO) (SAE 15W-40; 20W-40; 25W-40; 40).

This double specification is at insistence of heavy duty engine manufacturers who have required HSHT viscosity limits consistent with good engine durability in high-load, severe service operation. [They require an] HSHT value of 3.7 cP or [a] 27% more viscous oil at 150ºC (300ºF).

Yes, a 27% increase in viscosity makes a difference between Automotive engine that lasts 100,000 miles and a truck engine that lasts 1,000,000 miles!

When you consider that most automotive motor oils are ONLY 3 cP, while our
SAE 5W-50 SynLube™ Lube-4-Life™ Motor Oil has a rating of 5 cP, you can readily appreciate why we can claim [a] 300% to 500% increase in typical automotive engine durability, and that is with a substantial "safety" reserve!

See the entire section on SynLube Viscosity