Hi Carol,

There is some minimum amount of essential oil needed to fill the glass capillary tube such that the diffuser begins to work. Different oils will have different thicknesses, and different minimums. You'll have to experiment

As to pouring the 'used' oil back in the bottle, remember that the oil in the reservoir of the diffuser has not been affected in any way - and you may easily remove the orifice reducer from a bottle to make it easier to pour the oil from the diffuser back into the bottle from which it came - for use at a later date.