Some types of cooking oil benefit from refrigeration, while other types can be safely kept outside of the refrigerator. No oil should be kept near a heat source, like an oven or stovetop, or near central heating vents. It can be affected by both light and heat, and over time may become rancid. The oils that should be refrigerated include those with a lower saturated fat content, like safflower, sunflower, and canola.
The basic rule for storing oil is that those with higher saturated fat content, such as lard, palm or coconut, tend to be fairly stable. These can be stored outside of the refrigerator for longer periods of time. They should be stored in a dark place, not near a window with lots of light, since light can affect the quality of the oil.
Oil with a high polyunsaturated or monounsaturated fat content such as safflower, sunflower, canola or olive are much more delicate and should be stored in the refrigerator. Some argue that olive oil is not as useful when refrigerated, since it may become cloudy and may harden. Do not heat the olive oil in a microwave to get it back to a liquid state. Instead, simply allow it to stand out for a few minutes. When it returns to room temperature, it is still in good condition.
Still some tend to prefer not to refrigerate olive oil and claim refrigeration also affects taste. It may be advisable to buy it in small quantities if one tends not to refrigerate it. This will mean the oil is quickly used and runs less risk of becoming rancid. Be certain to store it in a cool dark cabinet, as this will extend its life.
Rancid oil should be discarded. They often have an unpleasant smell and a bitter taste. They may cause some stomach upset if used, and occasionally grow bacteria, though this is uncommon.
Oil that is being heated requires careful monitoring. When it starts to smoke, it is overcooked, making it essentially burned. Most oils will list a smoke point on the bottle. Some, like nut oils, have a much higher smoke point and may be better choices for deep-frying. Peanut oil is one of the best choices, as it does well at high heats.
Even with proper care, oil still has an expiration date, which should be observed. In general, one should throw it out after about six months, earlier if it has not been refrigerated. Before using suspected rancid oil, take a quick taste to be sure there is no bitterness.