I have been told that oil (especially olive oil) has an indefinite shelf life and will not turn rancid. I was under the impression that it would turn rancid, and have tossed oil whose age I cannot determine out of fear that it had "spoiled" and made anything I cooked with it taste funny. Please clarify.
No oil or fat keeps indefinitely. Exposure to air, light, heat and moisture -- probably in that order -- is the enemy of oil and animal fat. Olive oil, especially good-quality extra-virgin olive oil, will start to go rancid faster than soybean or canola oil.
Stored in a dark, dry cabinet, away from the heat of your stove, olive oil will stay fresh for about six months, or a year or more in the refrigerator. Regular cooking oil may remain good somewhat longer.
Use your sense of smell as well as sight to judge whether oil should be discarded. Also, what you will probably not use for salad dressing may be passable for cooking or frying. Fresh oil rarely goes so rancid as to "spoil."
Experts tell us to buy olive oil in small quantities. But for someone who uses a lot of extra-virgin olive oil, buying in bulk can make it much more affordable.
One problem with keeping olive oil in the fridge, however, is that it solidifies, or at least becomes very thick and cloudy. That is a problem if you want it for immediate use, as in a salad dressing.
The solution: We keep a small, tightly corked decorative bottle out on the counter, and leave the big bottle in the fridge. When it is time to refill junior, a half-hour out of the refrigerator makes the big container liquid again.