CAFE payments to the .Fed by the automakers are a real thing. Currently the automakers have to have an average of 54.5 MPG for all the vehicles they sell, but Trump has frozen (or is trying to freeze) that at the pre-Obama number of 37 MPG. If they fall short of that, they have to pay the .fed a "penalty" payment for non-compliance. They can fall short based on sales too, like if Ford sells more F150s and Explorers than their Focus, so there are a lot of factors at work.
Pumping the oil through an engine takes a surprising amount of power to do. It's a definite eye opener the first time you prime a pump when you build an engine and it is a definite drag on fuel consumption. So over the years the higher ups at the automakers have pulled rank over the design engineers and made them spec lighter and lighter viscosity oils and other fluids because they are easier to pump and have less drag. That's why if I look in the owner's manual for my F150 with a 5.4L I see that it says to use 5W30 or 10W30 oils, but yet they've since issued a TSB (technical service bulletin) that states that 5W20 will "yield better performance and durability" and that's what is to be used. It's a complete crock. The same vehicle with the same engine is spec'd to use 10W30 in Australia. Kind of odd. I run 10W30 in my F150.
The automakers are basically being "shook down" by the .gov and they do all they can to minimize the damage. FYI, be on the lookout, 0W16 weight oils are coming soon. And they'll be touted to be superior to anything else out there.