Tackling thinner and thinner motor oils seems to be the low-hanging fruit, in my opinion. I believe car manufacturers are reaching a point where they are doing this more to show they're actively addressing emissions and fuel economy rather than actually achieving significant improvements. The difference in net energy between two tanks of gasoline is greater than the fuel economy gains from switching from 5W-40 to 0W-16. However, the manufacturing cost increases exponentially because now you have to enhance precision, use more expensive materials, coatings, and so forth, to accommodate the thinner oil, which ultimately leads to a setback in long-term reliability.