Ingestible oil– both for eating and applying as skin care– can be made of nearly any part of plant, such as the seed, flower, or fruit. Essential oil is the strongest concentration of an extract, which is the most valuable due to the large volume needed to produce it. In ancient times, such as during those stories recorded in the Bible, often mention oil, which was used by the wealthy; any layperson, who owned a jar would’ve treasured it as something to be saved for their burial. Any oil that claims to be the same as a more expensive source is either of inferior quality (includes additional plant parts that don’t necessarily contribute the same– if any– liquid) and/or are actually a blend, which dilutes any properties of the oil you think you’re getting.
Each oil has its own properties; for our purposes we’ll focus on density, viscosity and smoke/ burn point.
Density of liquid refers to how much mass something has per volume; a fluid of light density will float on top of one with less density. Imagine a bottle of salad dressing before you shake it– the oil floats on top of the vinegar. Viscosity refers to liquid’s thickness. Obviously, oil is thicker than water or vinegar. The smoke point refers to the temperature at which an oil will burn. Some can withstand higher temps than others; it’s best to consider them along a spectrum. Likewise, different types of oils vary as to whether they’re a liquid or solid at room temperature and as to their required conditions to maintain the maximum possible shelf life.
Rancid oil is always a bad thing, not just because of the unpleasant smell and taste. Temperature, light and air (unsurprisingly microbes, also) can chemically alter oil. In some cases the change affects its beneficial properties. In other cases, rancid oil can turn into a completely toxic substance.
Some oil can help lower cholesterol but only in small dosages. All oil is high in fat, which takes its toll on internal organs, such as the liver, gallbladder and pancreas. Increased fat affects blood sugar. The more heavily an oil is processed (i.e. “refined” or “hydrogenated”), the more nutrients it has lost (just like fruit/ veggies), and the harder it is to digest and assimilate. Also, an imbalanced system can lead to insatiable cravings for excess sugar or salt.