Re: essential oils, the real ones should never be used neat, always diluted.
The crappy oils, sometimes pretending to be essential oils are often diluted already and have a short life before going nasty, or are actually fragrance oils which are not much use unless you just want something to smell, and it most likely won't smell of what it is meant to be.
People buy and use oils that they really shouldn't be using without advice, some of them can cause problems if used incorrectly and none should be used directly on the skin unless they are already blended with a carrier oil when purchased.
It is worth noting that if you want to use essential oils without at least reading a decent book on aromatherapy (by a qualified practitioner) you should be very careful of the quality and origin of what you buy, and be very aware of what it can do to you.
A few drops in a home made hand gel won't hurt but do check the properties first in case of allergies, a skin test is advisable and in the current situation tea tree might be the most sensible oil although lavender is also used for pretty much everything, but don't use too much, essential oils are pure oil (if they are genuine good quality ones).