The answer is that ANY one can be used regardless if your vehicle owners manual says to use, for example, a 5W-30. "W" means winter. In winter weather the 0W oil will flow like a 0W oil, and the 5W will flow like a 5W oil and a 10W will flow like a 10W oil just until the engine warms up. In order to understand the differences one has to first understand that the numerical values given to these various weight oils are strictly empirical numbers. For example, 0W does not mean that the oil has no weight. That is one of the reasons why we say it is strictly an empirical number.
In order to determine the differences between the three oils one has to look at the kinematic viscosity of each lubricant. The kinematic viscosity is essentially the amount of time, in centistokes, that it takes for a specified volume of the lubricant to flow through a fixed diameter orifice at a given temperature.
0W-30 is 57.3 cST @ 40 deg. C, & 11.3 cST @ 100 deg. C
5W-30 is 59.5 cST @ 40 deg. C, & 11.7 cST @ 100 deg. C
10W-30 is 66.1 cST @ 40 deg. C, & 11.7 cST @ 100 deg. C
As you can see from the data above the kinematic viscosities are extremely close. Therefore, whether you use the 0W-30, 5W-30 or the 10W-30 is strictly a matter of choice. With the small differences in kinematic viscosity you would be hard-pressed to detect these differences on initial engine start-up without specialized engine test equipment.
Any one can be used in a vehicle which requires either a 0W-30, 5W-30 or 10W-30 oil as well as in several other engine applications including an engine which recommends a 5W- 20 oil.
This leads to the next topic: many people also ask us if the 0W-30 is too thin a viscosity oil for high ambient temperature operation. The answer is absolutely not! Thicker viscosity oils are not always necessarily better since in addition to its' various engine lubrication functions, an oil must also effectively transfer heat. Only about 60% of an engines cooling is performed by the engine coolant, and only on the upper half of the engine. The remaining 40% of an engines cooling is performed mainly by the engine oil.
Although a vehicle that is recommended to use a 30 weight oil can also use a 40 weight oil, it is usually not needed. You will gain absolutely no benefit from using a thicker viscosity oil if it is not needed. The only time we recommend a 40 weight oil to a customer in a passenger car or light truck application is if the vehicle's engine is excessively worn and consumes oil at a higher than normal rate or if the vehicle is being used for very severe duty, high load, high temperature applications
I hope this brief discussion has answered your questions. If I have not sufficiently answered your questions or if you would like an additional explanation or recommendation for your specific application please email me.
good answer. To take a line from one of my favorite movies : "Check out the big brain on FL oil Guy" Thanks for the info. I knew the first number had to do with flow quality on start up. At least now I know 5w30 or 10w30 will be as good as each other.
but what about up north here where it gets to be -40deg C .in the winter :thumbdwn: 
