There are two main types of car oil - synthetic, a chemical lubricant, and crude, also known as conventional. There are synthetic blends as well:

1. Full synthetic oil has superior, longer-lasting performance from viscosity index to protection against deposits. This oil flows better at lower temperatures, but also maintains peak lubricity at high temperatures. Synthetic oil is more expensive than other types of motor oil, so make sure your car really needs synthetic before purchase.
2. Conventional oil is organic and highly reactive to fluctuations in temperature. You can find this type of oil in multiple viscosities depending on the climate in which you live.
3. Synthetic blended oil is a combination of conventional and synthetic oils, and is less expensive than fully synthetic. This version still provides the performance enhancement of the synthetic, however. The blend is recommended for heavier loads and higher temperatures. It is less volatile, evaporates less and reduces oil loss, increasing fuel economy in the process. The price difference between the blended oil and the conventional is negligible.http://www.driverside.com/auto-library/what_motor_oil_is_best_for_your_car-50 http://www.pnwautos.com/content/shared/?cid=88&eid=3698 http://www.popularmechanics.com/cars/how-to/products/1266801 

5W-30 is the most common oil. If you are unsure what type of oil your vehicle needs to run, consult your car's Owner's Manual or maintenance guide.http://www.youtube.com/watch?v=qiMAHyyJ3FE&feature=player_embedded#at=47 

All varieties of motor oil are graded by the Society of Automotive Engineers (SAE) and rated by the American Petroleum Institute (API). The grades range from 0-70 where the higher the grade, the higher the viscosity of the oil. The age of a car determines which viscosity it needs, the rule being the older car, the thicker the oil. Since motor oil becomes thicker in colder temperatures and thinner in warmer, oil that meets low temperature requirements get a "W" after their grade. This being the case, consider the climate you normally drive in when selecting your motor oil.http://www.driverside.com/auto-library/what_motor_oil_is_best_for_your_car-50 http://www.aroundhawaii.com/lifestyle/art_and_leisure/2009-06-choosing-the-right-oil-for-your-car.html 

The number following the "W" in your oil represents the viscosity of the oil. The more resistant the oil is to thinning in the presence of warmer temperatures, the higher the second number following the grade and temperature. So, 10W-40 will have a higher (thicker) viscosity at hotter temperatures than 10W-30, for example.http://www.popularmechanics.com/cars/how-to/products/1266801