Conventional vs. Synthetic
Traditionally, manufacturers made conventional engine oils from a mineral base oil stock derived from crude oil. Synthetic base oil stocks are produced by either highly processing and refining mineral based oil stock, or by chemically engineering synthetic base oil stock. Current additive packages blended into conventional oils allow them to function longer than oils of the past, but it is primarily the synthetic base stocks that allow greatly extended intervals.
Synthetic oil offers many advantages in durability versus conventional petroleum oil and it is all based on the synthetic oil’s superior resistance to heat and oxidation while requiring little or no viscosity index improvements, which are the most vulnerable to degradation as oil ages. Synthetic oil is more accurately controlled at the molecular level and therefore can be engineered to resist the forces in an engine that work against oil durability.
Ultimately the choice is up to you. But hopefully some of the information you have received here, and you that you can find on this website: http://auto.howstuffworks.com/synthetic-conventional-motor-oil.htm, will help you make the decision about which is best for you and your car.