I like the way that Chevron-Texaco's website puts oil weights into a historical context, as well as a use context: "SAE 10w-30 is recommened for many older vehicles and a few late model cars and trucks, often those with larger 6 cyl. and V-8 and/or high performance engines. SAE 10w-30 provides improved fuel economy relative to heavier visciosity grades."
"SAE 10w-40 is still a very popular viscosity grade in warmer climates, thought it is not recommended by most auto manufacturers for their late model North American vehicles. It is often recommended for older model cars that see regular high temperature operation." They also have a few rhymes for 5w-20, 5w-30, and 20w-50 weights as well.