Lately I have read a lot on these new (introduced in early 2000s) oil types like 5w20 and 0w20
There is a myth of economy revolves around these oils .. I so far, strongly disagree on it, based on a theory
Start reading from post #3 .. skip what is posted below
First a few terms
CAFE : Corporate average fuel economy
EPA : united states environment protection agency
Foot Print : the product of multiplying a vehicle's wheelbase by its track width
Relevant links
https://www.sae.org/publications/technical-papers/content/2012-01-1618/preview/
https://www.swri.org/sites/default/files/sequence-vid-test.pdf
https://www.truckinginfo.com/149645/research-shows-oil-viscosity-affects-fuel-economy
Chevron installed equipment to conduct fuel economy testing using the Volvo D12D Fuel Economy Test, a lab-based test used in Europe, in its lab in Rotterdam in The Netherlands. This kind of testing allows researchers to precisely load the engine and run it under prescribed conditions the computer controls.
"Many of the OEMs have internal fuel economy tests, but most are not willing to release theirs because they are viewed as a competitive advantage," explains Gary Parsons, global OEM and industry liaison manager, Chevron Oronite.
The testing used a 15W-30 as a reference oil. The results were "weighted" with factors to represent hilly conditions or flat conditions.
In flat conditions, where obviously you would get the best fuel economy no matter what the oil, a 15W-40 oil performed nearly 0.8 percent worse than the base 15W-30. The 10W-30 performed nearly 0.2 percent better than the 15W-30. That's close to a 1 percent difference between the 15W-40 and the 10W-30.
Julie Madubost, Hiroshi Watanabe, Gary M. Parsons
ISSN 0350-350X
GOMABN 49, 3, 250-282
Stručni rad/Professional paper
UDK 665.765:006.83(100):621.434:629.331:656.138.
NEW GASOLINE PASSENGER CAR ENGINE OIL
PERFORMANCE CATEGORY
page 278