There are many different types of oils on the market. They all offer certain amounts of protection, varying price, variable viscosity, and different levels of efficiency. Their main purpose is to be used as a lubricant, but also as a way to cool engine components. I am going to try and explain as simply as possible the current types of engine oil that get used in the automotive field.
All oils have to meet certain standards set forth by either the Manufacturer or the American Petroleum Institute. The newest API standards are SN. One of the newest manufacturer standards is Dexos. Yes, Dexos oil is not a brand, a type, or a specific synthetic, it is mearly a new standard GM has released.
Dexos approved oil barely beats SN standards, but to be Dexos approved you have to pay GM for the branding. Kind of a shame, isnt it? The used of non Dexos oil will not void your warranty unless the dealer can specifically prove that using a non approved oil actually caused the problem.
Synthetics are man produced oils derived from modified petroleum elements or other raw ingredients and contain other chemical ingredients in the form of a additive pack. They offer superior protections in all climates and engine operating conditions. Synthetic oils have several advantages and only one disadvantage - cost. They are extremely pricey when compared with natural or blend products.
Synthetics generally last longer. They do not break down as quickly as regular oil. Also, their molecules are all exactly the same size and conventional are not. That is the theory behind why switching to synthetic on an old vehicle will cause excessive leaks as the different sized molecules wont clog up the hole - but recent studies have determined there is little effect. I do not condone this statement, but rumor has it that "Synthetics have been rumored to last for several thousand miles (example 12K) as long as you change the filter."
Another slight disadvantage of synthetic oils.. they must not be used in a brand new engine during the break in process. You must use conventional oil. Usually the break in process is completed by the vehicle manufacturer during exit from the factory and during removal after shipment. After the first oil change, conventional oils is no longer needed.
Regardless of the type of oil used, always change them regularly based on manufacturers recommendations. The additive packs in oils are what fails after a short while.