Simple…. If your car manufacturer specifies synthetic oil, you have no options. You MUST use synthetic oil. You will damage your engine if you use conventional oil when the manufacturer specifies synthetic oil. One of the advantages of synthetic oil is that you only need to change it every 6 months/6,000 miles.
If your car’s manufacturer does not specify which type of oil to use you can safely use conventional oil or synthetic oil. I read lots of trade journals, and I have never seen a report which says synthetic oil is better for your car than conventional oil if your car’s manufacturer says your car can use conventional oil.
If your car is designed for conventional oil, you must change your oil every 3 months/3000 miles. If your car is designed for conventional oil and you use synthetic oil you still need to change your oil every 3 months/3,000 miles. Hence, using synthetic oil will cost you more money, but it does not give you any benefits.