I've noticed that the recommended motor oil weights for cars in the last 5-10 years are often lower than earlier models. It seems like 10w-30 used to be almost ubiquitous in the 80s and early 90s, but now I'm seeing manufacturers recommend 5w-20 and 0w-10.
What does this change imply? Could it indicate improved engine design or manufacturing techniques? Tighter tolerances?
Or is it related to something else?