I can't swear to it, but I believe the second number shows the temperatures it can withstand (ie 10W30 would be good down to 30 degrees).

The viscosity

It is a measurement of how quickly a certain amount of oil flows thru a small opening at a particular temp. The first number is a measurement at a cold temp and the second (30,40,50) is a measurement when it is much warmer. So, 40 is a bit thicker than 30 when it is warmer. The values have nothing to do with any value of temperature themselves.

10w-30 / 10w340 < multi grade oils. It used to be that in vehicles a single grade oil was used: meaning something like 30, 40, 50 weight motor oil Single grade some people still use in old old cars that used it back in the day if.....and lawn mowers and in hot climates where multi-grade oils arent needed. Then the multigrade motor oil was made.....multi grade means: a different viscosity (thickness)when cold & hot. *Why use a multi grade oil? Because the first number 20w / 10w /5w / 0w is the viscosity of the oil at startup/ cold temperature which lubricates better on start up getting to places the thicker oil wont on startup - when the motor starts to warm up then you have the second number eg. if your oil is a 10w-30 the motor starts up with the oil thinner at 10 weight and as it warms up to operating temperature its thicker to 30 weight. The reason it has thicker oil is to handle the heat etc. Now on a 80Farenheit day / 30 degree C day that thickness may change a bit but it still thinner than the 30 at startup. COMMON MISUNDERSTANDING - eg. 10w-30 DOES NOT mean 10weight - 30 IT MEANS 10Winter - 30 that first number is what makes it a multi grade oil. Places like california and arizona for a long times didnt use multi grade oils (going back to 70s here) but they do now. And i'm sure there are people who still use a single 30 weight out there in there car. It's more common than in eastern North America where we have winter so it makes sense to use a multi-grade. But use what it says on your oil cap or in the manual -if you don't know go tot the dealer and ask/ call ask. Really only when you use a thicker oil that specified is if your car is burning oil excessive, in most cases for the average joe blow car. Racing/ sports car is another matter. Also today they build engines with closer tolerences so if you were to throw a 20w-50 or a straight 50 oil in something like todays modern honda civic or ford explorer unless the engine was just worn and burning oil like mad you would starve the engine of oil because it would be to thick. * So if you took like a 5w-30 or 5w-20 oil and put it in something like a 68 Pontiac GTO or a 67 Ford Falcon it would likely just leak out becuase the tolerences and fit and fish of the engines werent as tight/close then. Remember to use what the engine call for when it was made. Alot of people put multi grade synthetics in really old cars that used single grade in the day and they here tapping the lifters just rattling away ....becuase they didnt use what the engine was designed for. And going back to the 80's most cars used 10-40/ 10-30 and even 5w-30 and that started into the 90s 5w-30 was more common than in the 80's not just for use in the winter but all year. Because a lot of people use 10w-30 rest of the year and 5w-30 winter (it says u can in most manual) But engines started being built with tighter tolerences and 5w-30 just as a regular all around oil was more common plus it was better for fuel economy than. (this is also temperature dependent. in california arizona is gonna be differnet than Boston,MA or Ontario Can) So coming to the 2000's Honda started the trend that Ford has followed in using 5w-20 motor oil even thinner some even use in really cold -degree temp. 0w-20. But new Honda Civics, '08 Ford Focus use 5w-20. Just a few years ago it was 5w-30 being the oil that was thought of as thin becuase its been advertised for well over a decade as winter oil. So its hard for people to get their minds around a lot of this as the industry changes and specs change. It's easy to get confused and a lot of people do and even places like jiffy lube have been known to put in the right oil thats why it's good to understand the difference. Becuase if your off on the highway and stop at a service station and check ur oil and realize ur a litre low its good to know what to use & why ur using it.