“The aim of the oils is to encourage your body to balance itself, and not to treat the symptoms. In its gentle way, the oils encourage your body to build its own immunity, and also strengthen its own immunity. So, as your body gets stronger, you will find that you don’t need certain oils any longer.”
Why using several different oils can be far more effective than only one?

Patricea Chow-Capodieci: Your body takes whatever it needs from the oil AT THAT MOMENT, and dispels whatever it doesn’t need through perspiration and urine. That is why Frances Fuller always says to use only one drop each time and frequently, rather than many drops infrequently. So this means there is no build up of the oils in our body. And that is also why we need to drink more WATER to help with the process, and not drinks like coffee or tea which actually dehydrate the body more.
Actually, layering the oils will work better than rotating the oils. Again, works with the same principle of body taking what it needs at that moment. Each oil has more than 200 constituents and we don’t know which will react with our body, since everyone is different. Layering the oils gives your body more choices.
Read: LAYERING OILS

Frances Fuller: I DO believe that it is better to layer ONE drop of several oils FREQUENTLY for maximum effect – AND that the body does NOT “become immune” to the effect. Rather, it becomes BALANCED. That is why using several different oils can be far more effective than only one – you are offering the body a “huge buffet of molecules” from which to choose for its balancing. It will eliminate whatever it does not need.