0W-20 runs in the American motor in order to barely eek out another 1MPG to barely meet the CAFE standards necessary to ship product. 5W-30 runs in the EU motor because it’s the best viscosity for the damn engine. Which they determined long ago with engineering and testing, both in lab and real world results.
Now pull those engines apart after 100K miles and see why we need to get rid of CAFE bullshit. The American environment isn’t being “saved” by forcing Americans to replace their disposable cars before the fucking loan is even fully paid.
The problem is not the CAFE standards per se. The problem is that they are trying to meet them by playing tricks instead of with actual design changes. And even though they come with thinner engine oil, 90% of people will put standard oil into the cars at the first change. So they get higher MPG for the first 5,000 miles, and then the same as they did before the CAFE standards.
I suppose that in theory, you could argue that the standards are flawed for not requiring normal oil weights during testing, but that's about it.
The correct way to hit the MPG targets is to use more electric drive trains. If you make your cars hybrids, you can improve the efficiency of the electric drive train and find ways to engineer the vehicles to weigh less by using more modern materials, and you won't have to work too hard to hit the standards.
Better yet, push actual EVs, which typically have an eMPG of 100+. If just one-third of your vehicles get 130 eMPG and the rest get 30 MPG, you're averaging about 53 MPG already.