See Figure 1
Oil must be selected with regard to the anticipated temperatures during the period before the next oil change. When buying oil for your vehicle select the oil viscosity for the lowest expected temperature and you will be assured of easy cold starting and sufficient engine protection. The oil you pour into your engine should have the designation "SG" marked on the top of its container.
There are many excellent synthetic and fuel-efficient oils currently available that can provide better gas mileage, longer service life, and in some cases better engine protection. These benefits do not come without a few hitches, however _ the main one being the price of synthetic oils, which is 3 or 4 times the price per quart of conventional oil.
Synthetic oil is not for every car and every type of driving, so you should consider your engine's condition and your type of driving. Also, check your car's warranty guidelines at the dealership that you purchased the car from, regarding the use of synthetic oils and your powertrain and or extended warranty.
Both brand new engines and older, high mileage engines are the wrong candidates for synthetic oil. The synthetic oils are so slippery that they can prevent the proper break-in of new engines; most manufacturers recommend that you wait until the engine is properly broken in (5,000 miles) until using synthetic oil. Older engines with wear have a different problem with synthetics: they use (consume during operation) more oil as they age. Slippery synthetic oils get past these worn parts easily. If your engine is using conventional oil, it will use synthetics much faster. Also, if your car is leaking oil past old seals you'll have a much greater leak problem with synthetics.
Cars used under harder circumstances, such as stop-and-go, city type driving, short trips, or extended idling, should be serviced more frequently. For the engines in these cars, the much greater cost of synthetic or fuel-efficient oils may not be worth the investment. Internal wear increases much quicker on these cars, causing greater oil consumption and leakage.
The mixing of conventional and synthetic oils is not recommended. If you are using synthetic oil, it might be wise to carry 2 or 3 quarts with you no matter where you drive, as not all service stations carry this type of lubricant.