Is it true that everything that has been cooked in oil is unhealthy? What some say is that oil gets very hot when heated, and ruins anything cooked in it. Even the structure of the healthy oils itself also is destroyed and changed into something unhealthy when heated. Others say some oils (such as coconut oil, olive oil, butter etc) are very resistant to heat and can be used for cooking.
Obviously canola oil is not suitable for human consumption even if it's not heated, according to some. Olive oil, rice bran oil and grape seed oil are often called the "healthy oils".
I personally never cook anything in/with oil not because of health concerns, but simply because oil splashes, evaporates and makes a mess in the kitchen as a result. I only pour room-temperature oil (mostly olive oil) on my salads.