Once the packaging of oil is opened, it should be used within two to three months (because most of the oil is antioxidant-free, so once the packaging is opened, it will get oxidized). 

Oil can be stored in refrigerator which makes it keep a bit longer. The clarity of peanut oil and olive oil may be affected but it will not affect its flavor and quality. Used oil can't be poured back to the oil bottle. Also, used oil cannot be stored for long time because the seasonings, water, protein, etc. of the food have been mixed in the oil. These substances are harmful to health.

According to an article published by USDA in 2016, male is suggested to consume 6-7 teaspoons of cooking oil per day; women should consume 5-6 teaspoons of cooking oil per day; and children should consume 3-4 teaspoons of cooking oil per day.

The reused oil has been mixed with fat, seasonings, water, protein, etc, these substances will degrade the oil quality. It is reported that consumption of reused oil will lead to health problems, like diabetes, heart disease and increase the chance of cancer. Therefore, used oil should not be used repeatedly.

Normally, the shelf life of unopened edible oil is two years. With no antioxidant added to oil, it will combine with oxygen in air after opening and aroma will be changed after long time. Therefore, if you do not consume much oil, you should buy a small package size. After opening, you need to use it within two months for quality. 

If the packaging of oil is opened for long time, you should pour the oil into a bowl and smell it. If there is no weird aroma, it can be used, but it is recommended to consume it as soon as possible.

Opened oil is better to put in a refrigerator because it can extend the freshness of the oil. The color of peanut oil and olive oil may change or even congeal at low temperature environment. This is normal and it will be back to liquid state at room temperature.

There's no problem to consume a single type of oil if the diet is balanced, because there are different kinds of fatty acids in the food. However, if you have unbalanced eating habit, you should consume different types of oil to gain appropriate fatty acids. For example, canola oil can provide Omega-3 to people who do not eat fish. Peanut oil provides essential fatty acids to people who cannot eat nuts.  

Generally, it depends on the type of cooking. For example, if you are cooking Chinese dishes or steamed fish, you can choose peanut oil with rich aroma. You can use canola oil for fried food and corn oil for salad dressings. 

When the oil is heated and start to discharge smoke, we should put in food. This will not cause cancer, especially if the oil is fresh. However, don't overheat food for a long time till white smoke appears because overheating will increase the chance of cancer. At the same time, we should avoid to reuse cooking oil because of its high impurity and may have carcinogen. In addition, if the air ventilation system does not function well, the polluted air is carcinogenic.

Harvard University announced a new food pyramid. The traditional food pyramid put oil on the top layer which recommends people to eat less oil. We should definitely eat less animal oil, palm oil, coconut oil or other oil with high saturated fatty acid. However, we should consume proper amount high quality vegetable oil like  peanut oil, corn oil, canola oil, and olive oil, etc.. Therefore, Harvard University announced a new food pyramid that high-quality cooking oil was put on the second layer with the bottom layer as sports. Many people think that cooking oil can only improve the taste of food, but with high energy level. In fact, the basic function of oil is to provide useful fatty acids and to bring oil-soluble vitamins A, D, E, K, etc. to the body. If people do not consume oil, it will lead to a series of diseases like cardiovascular, liver and kidney problem, joint problem, hormone imbalance, etc.. Also, it influences the brain and vision development of children and foetus.

Everyone knows that eat too much oil will contribute to weight gain. Excessive carbohydrates will be stored in our body in the form of oil, which will raise blood glucose level. 
Excessive consumption of carbohydrates (such as desserts) has even worse effect than excessive consumption of cooking oil.

The produced oil should be good oil as long as raw materials are from fresh and non-polluted source, factory is well-equipped, and with international accreditation such as ISO22000, ISO 9001, HACCP, etc.. Consumers should choose according to their own requirements. Cold pressed oil or refined oil has their own benefits.

Peanut, olive, etc. is the best when cold pressed. It contains many good elements such as plant polyphenols, natural pigments which have antioxidant function, and rich in natural vitamin A, E, K. However, if the crops are stored for long time after harvest, it will be infected with mold that produces toxins such as aflatoxin, which is a carcinogen and must be removed by refining at high temperature.
Some plants are not suitable for cold-pressed even if they are fresh. Soybeans, rapeseed, cottonseed, etc. all are with natural toxins, so must be refined at high temperature to remove toxins.

Manufacturers generally press, then use the solvent to extract the remaining fat from oil cakes. Boiling point of solvent is at 40°C. The refining process heats up to 250 ° C. All odors, solvents and toxins are removed under vacuum condition at high temperature. Therefore, the quality control of refinery process is very important.

Bad oil refers to the fat that is not good for human health. Oils with high saturated fatty acids such as coconut oil and animal oil, etc. are not good for heart health.

Mold oilseeds or contaminated edible oils are not good for health. Consumption of oil with residual ochretoxin increases the risk of liver cancer.

Preservatives, artificial colors, artificial flavors, artificial antioxidants, etc., are additives that can easily reduce immunity and damage the metabolic system. These additives, although within safe dosage range, will cause different degrees of damage to the body after accumulation in long term.

Oxidized oil
When edible oil is repeatedly heated, oil will be deteriorated, and easily generate harmful carcinogens, such as free radicals which cause damages to health. In addition, improper storage of oil will also cause oil oxidation.

No single oil is the best for different aspects. Monounsaturated fatty acid of olive oil is too high that pregnant women or children should not consume this single type of oil in their diet. Olive oil is in lack of essential fatty acids which are essential for brain, vision and hormones development. Omega 3 in fish oil is extremely unstable and easy to deteriorate. If we keep balanced diet, there should be no issue, because there are different level of fatty acids in food, otherwise, it is better to get fatty acids from different oil type. For example, canola oil provides omega 3 to people who do not eat fish, peanut oil provides essential fatty acids to people who cannot eat nuts.

Peanut oil, corn oil, canola oil, soybean oil, sunflower oil, olive oil, etc. are common edible oils. When we consume different type of oil, fatty acid and nutrients reached a balanced state which is beneficial to health. Conversely, long-term consumption of a single type of oil is not beneficial for maintaining a balanced diet. Therefore, it is recommended to consume a variety of oil and change regularly.

As human beings need a balanced mix of nutrients and fatty acids, so it is recommended to switch oil use over time. A long term usage of a single oil type is not good for maintaining a balanced diet, so iti is better to have diversified oil usage. Peanut oil, soybean oil, canola oil, corn oil, sunflower oil, olive oil, etc. are commonly used edible oil.

Consumers can refer to the manufacturer's quality certification. The issuing authority will strictly monitor the quality of the products from the selection of raw materials, the refining, storage and filling of intermediate oils, storage and transportation of finished products. ISO22000 food safety management system certification, HACCP food safety management system certification and ISO9001 quality management system certification, etc. are good examples of those.

Smoke point of oil varies among different oil types. The smoke point of refined oil is higher than extra virgin oil. Oil with high nonesterified fatty acid is with a low smoke point.

Oil with low smoke point:
Extra Virgin Olive Oil  ~130°C
Coconut Oil ~160°C
Butter ~150°C
Oil with low smoke point is suitable for salad dressings or cooking at low temperature.

Oil with high somke point:
Canola Oil  ~230°C
Corn Oil ~220°C
Peanut Oil ~230°C
Refined Olive OIl ~240°C
Oil with high smoke point is suitable for all knids of cooking, like pan fry, stir fry, deep fry, etc..

Excessively heated or re-used cooking oil is always mixed with food condiment, protein and carbohydrates. These impurities accelerate the oxidiation, rancidification of oil. It is very likely to bring carcinogens to oil which is harmful to health.

Fresh cold pressed peanut or olive oil is high in polyphenols, and natural pigments which are antioxidants, rich in  vitamins A, E, K, etc..
However, if the crops were stored for a long period of time, they will get infected by certain fungi and produce toxins such as aflatoxin, so must go through high temperature refining to remove it. Not all fresh plants are suitable for cold pressing. For example, soybeans, rapeseed, cottonseed, etc. as they contain natural toxins, so must be refined at high temperatures to eliminate toxins.

Manufacturers generally press, then use the solvent to extract oil from oil cakes. Boiling point of solvent is at 40°C. while the refining process heats up to 250°C. All odors, solvents and toxins are removed under high temperature environment and vacuum. Therefore, refined oil is also hygenic and good for health.

The oil contains natural pigments which are derived from oil-making raw materials, e.g. peanuts with carotene and corn with zeaxanthin, which is the reason of color difference between different types of oils. Even with the same oil, the harvest time and different processing methods will affect the color of oil. In addition, when the carotenoid-containing oil is exposed to ultraviolet light, the color will turn lighter, so the color differnece of the oil is a normal and is a natural phenomenon. There is no directly relation to the nutrition value and quality level.

Harvard University announced a new food pyramid. The traditional food pyramid put oil on the top layer which recommends people to eat less oil. We should definitely eat less animal oil, palm oil, coconut oil or other oil with high saturated fatty acid. However, we should consume proper amount high-quality vegetable oil like peanut oil, corn oil, canola oil, and olive oil, etc.. Therefore, Harvard University announced a new food pyramid that high-quality cooking oil was put on the second layer with the bottom layer as sports. Many people think that cooking oil can only improve the taste of food, but with high energy level. In fact, the basic function of oil is to provide useful fatty acids and to bring oil-soluble vitamins A, D, E, K, etc. to the body. If people do not consume oil, it will lead to a series of diseases like cardiovascular, liver and kidney problem, joint problem, hormone imbalance, etc.. Also, it influences the brain and vision development of children and foetus.

Everyone knows that consume too much oil will lead to weight gain. Excessive carbohydrates will be stored in our body in the form of oil and raise blood glucose level. 
Excessive consumption of carbohydrates (such as desserts) have even worse effect than excessive consumption of cooking oil.

Pure oil is simple in nature, such as extra virgin olive oil with low smoke point, so not suitable for high temperture cooking. Low in essential fatty acids, so not suitable for children, pregnant women or breastfeeding moms for long-term consumption. Olive blended oil provides a more balanced proportion of fatty acids which is cheaper than pure oilve oil and suitable for all kinds of cooking.
There is no research to support that claimed the blended oil is harmful to health. Conceptually, if there's no additional processing or additives added during the process of blending, it is also suitable to consume.

Olive oil helps reduce bad cholesterol level in blood and promote cardiovascular health. It contains antioxidants, like polyphenols and vitamin E. Monounsaturated fatty acid of olive oil is too high that pregnant women or children should not consume this single type of oil in their diet. Olive oil is in lack of essential fatty acids which are essential for brain, vision and hormones development. Vitamin K of extra virgin olive oil is relatively high, people who need to take anticoagulants should not use olive oil in long term.

Although coconut oil is a kind of plant oil, it contains over 90% of saturated fat which will increase the total cholesterol level and the risk of cardiovascular disease. 

This is a fallacy, peanut oil does not contain cholesterol. Peanut oil is similar as olive oil and canola oil, contains a lot of monounsaturated fatty acids which helps to reduce low density cholesterol, i.e., bad cholesterol. At the same time, it does not affect high density cholesterol in the body, which is commonly known as good cholesterol.

The rumour about canola oil is harmful to human body has been spread over years. The Centre for Food Safety (CFS) of the Food and Environmental Hygiene Department and Consumer Council has publicly clarified that canola oil is a healthy cooking oil in 2010. Currently, canola oil is widely used in Europe, America, Japan, Australia and New Zealand because its saturated fatty acid is the lowest compared with other common cooking oils. Also, it contains a high level of monounsaturated fat, which makes it is more stable and not easy to oxidize. Moreover, canola oil contains omega-3 fatty acid which reduces bad cholesterol (low density lipoprotein cholesterol) and improves the proportion of good cholesterol level that helps to reduce the risk of preventing heart disease.

Oils with high level of saturated fatty acids may cause cardiovascular disease.
Coconut oil (90%), palm oil and animal oil are high at saturated fatty acids (50%) which will raise total cholesterol level.

Oils with high level of  monounsaturated fatty acids are classified as "good oils".
Monounsaturated fatty acids can lower bad cholesterol level while increase the proportion of good cholesterol level. Vegetable oils such as canola oil, peanut oil, olive oil, tea seed oil, rapeseed oil are good oils that may help to prevent people from cardiovascular diseases.

Polyunsaturated fatty acids are also known as essential fatty acids which must be taken from food. It can help reduce total cholesterol level. Canola oil, peanut oil, corn oil, soybean oil, fish oil, etc. are good examples of oil with high level of polyunsaturated fatty acids.

Omega-3 fatty acid is a kind of polyunsaturated fat which is an essential fatty acid for human beings and cannot be produced by human body. Therefore, human must get omega-3 fatty acids from food, like oil from fish, seaweed, soy, canola seed, walnut, etc.

Omega-6 fatty acid is a kind of polyunsaturated fat which is an essential fatty acid for human beings. It exists in any kind of animal or vegetable oils.

Is an essential structural component of all animal cell membranes that regulate somatic function.

A class of lipids that is a major component of all cell membrane. It allows body's lipids to dissolve in water and assists the transmission of glucose, electrolytes, carbon, and carbon dioxide among our cells.

Nonesterified fatty acid accelerates rancidity of oil.

Triglyceride is stored between  liver and fat cells as a backup energy. When used as energy, it breaks down into nonesterified fatty acid and flows into the plasma.

Trans fat is mainly produced during the hydrogenation of vegetable oils. The effect on health is even worse than that of saturated fatty acids. It increases the level of low density lipoprotein cholesterol while reduces the high density lipoprotein cholesterol in blood.