I just bought a new car last month. My previous car was a 1997 model and it specified 10w-40 motor oil, as had every car I have owned for the past 30 years. This new car (a 2005 Ford Focus) specifies 5w-20 oil. I've since found out that 5w-20 has been pretty much standard for the past couple of years.
Question: Why the apparent reduction in lubricating requirements? I can understand that theoretically the 5w requires less power to pump and therefor *might* result in some mileage improvement; but the high temp reduction from 40 equivalent to 20 equivalent doesn't make sense to me.
What am I missing?