Oil becomes less effective as it ages, and excess of moisture and friction products that forms in the engine lead to shorter engine life.
Many modern cars have a service reminder monitor that alert owners when to change oil. If you don't have automatic reminder, check your owner’s manual. Also we recommend to checking your oil level at least once a month. Be sure to visit service at the first sign of a oil leak.
Oil change interval vary by vehicle manufacturer. Usually it is 3,000 miles for conventional oil and 5,000 for synthetic oil. A prolonged oil-change interval will cost you more money with each oil change, as the synthetic oil change price is more expensive.
Read your vehicle owner’s manual. It is unnecessarily to buy synthetic oil if it is not prescribed by car manufacturer.