Q&AHow many of your favourite oils can you mix together before you lose the plot, so to speak? And are, say, three stress-relieving oils blended together more effective than one on its own?

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If you want something for, say, aching muscles or sinus congestion, then “fragrance impact” is less important. However, if you are looking for an ideally-proportioned, wonderfully-smelling blend, most people find that if they mix more than 4 or 5 oils together they begin to, as you say, lose the plot.
You have to remember that each essential oil is composed of some 50 to 100 aromachemicals, and so when you blend three oils together you are actually blending a couple of hundred fragrance materials. Blending many oils together successfully is possible, and I routinely use between 10 and 20 oils in blends for products. However, the relative quantities have to be very precise, and losing the plot is, if anything, much easier.
Three stress-relieving oils blended together are probably going to be more effective than a single oil, maybe not hugely, but blends do tend to be more effective. I think if you achieve “fragrance synergy” then you are probably also going to achieve therapeutic synergy. Plus, whoever chooses the oils and makes the blend puts their positive energy into it, so probably it will have more power than any single oil.