You’re correct in that 0w-16 isn’t going to bump your mpg from 36 mpg vs 35 mpg. The numbers I’m about to mention are just for illustration purposes only. Let’s say by using 0W-16 oil, the vehicle gets .001 mpg better compared to 0W-20 . Obviously the vehicle operator won’t notice such a minuscule increase, and it’s not enough to raise the MPG a full MPG. However, when you sell a million vehicles in a given year, that .001 mpg increase per car adds up to a lot of saved CO2 and barrels of oil from being used. Then consider the fact that 5W-20 has been factory fill since around 2001 for a few automakers, assuming those vehicles were serviced with 5W-20 their entire lifespan, that is a large amount of CO2 being prevented from being released into the air.