To know if essential oils are pure, not diluted with vegetable oils or mineral oils, a simple test is to put a drop or a few drop on paper and let it to dry. After the EO is dried, it don't have any oily trace on the paper, just paper slightly colored from the color of the EO , if the EO is mixed with some kind of oil, it will leave a translucent mark when dry.
Essential oils are surely nice to mix with conditioner cream, wax or oil to achieve some purpose, like adding antiseptic, anti-fungal properties to saddle, maybe. Regarding scent, a fun project will be to recreate natural scent of leather as chemicals manufacturer produce in synthetic way (they do it also for pencil: very good brand of pencil always have a very nice scent of dry wood compare to cheap brand. It's perfumed pencil. They do it also with leather and many other things).
If you are interested by essential oils, their scent, properties and even magical uses, you can download a free software:
Aromapedia
My link
or
My link 2
It's not a professional use software but it have some nice info.