oil "weights"??

Something that always confuses me....

Lets say you have a quart of oil that is 10w-30. What does the 10 and the 30 mean?

I'm still sleepy

The "W" stands for winter, so 10W indicates that the oil will be the viscosity of a 10 weight oil when it's cold. The 30 indicates that the oil will be equivalent to a 30 weight oil at 100 C. The higher the number, the thicker the oil. For winter use you want an oil that can pump at very low temperatures like 5W30 or even the new 0W20. In the summer you want a thicker oil like 10W40. It's ok to vary the weights by one grade up and down (ie 5w30 vs 10w40), but it's usually not good to vary by 2 grades (5w30 to 15w50). The thinner oils will give more mpg than thicker oils. Thicker oils will be harder to pump, and not give as good mpg but they will coat surfaces with a thicker layer than a thin oil. Synthetic oils are known for being able to flow at much colder temps than petroleum oil, and they also withstand high temps better too.

when 0W-20 comes out, don't ues it right away if you plan to do so. It is going to have a new classification system, and might mess up your engine.

Sciguy Jim is right, but don't use 10w-40 unless you car is made in the mid 80's or older. It hasn't really been reccomended since then

Right. I just used those weights as examples, not actual suggestions for use.