Best oil info I've seen is from a guy named Dr. Haas on the Ferrari Chat website: http://ferrarichat.com/forum/faq.php?faq=haas_articles
Each number in the oil viscosity designation refers to a particular weight when cold (i.e. during cold startup, which can be summer also!) and when at operating temperature. Generally the higher the number, the thicker the oil. However each number CAN NOT be directly correlated to each other!
So for example 10w30 and 10w40 will be about the same viscosity under about 100 degrees or so, but the 10w30 will be thinner at operating temperature of 170-190 degrees.
Similarly, 5w30 will be thinner than 10w30 when cold, but both will be about the same at operating temperature.
However, a 5w or even a 0w will be much thicker at cold startup than even a 20-weight oil like 5w20. This means the oil has a more difficult time getting in between bearings at initial startup, which also means that around 80% of engine wear happens within the first few minutes of starting a cold (under 100 degrees) engine.
For this reason, it's really best to find the oil with the thinnest cold viscosity that still has the correct operating temperature viscosity. This will help reduce engine wear that occurs during cold startup. This is why I switched my Acura Integra GS-R with over 300K miles over to Mobil1 synthetic 0w-30 (factory calls for 5w30, or 10w30 if climate doesn't get too cold). Been using it for a couple of years now and the car still averages 32 mpg. The car went nearly 200K miles using Valvoline MaxLife 5w30 synthetic blend before that.
If I ran a yard business I would probably do the same - use the oil with the thinnest cold viscosity that still has the correct operating temp viscosity. I know Mobil1 has 0w-20 and 0w-30. Not sure if anyone makes a 0w-40.