Castor is very heavy and can cause build up when applied directly. If you use a carrier oil (I recommend sweet almond has it's lighter and also good for the scalp) then this helps greatly and it easier to wash out. You can drop a few drops of rosemary or peppermint oil in the mix to help with the growth and promote circulation. I've had wonderful results. Didn't know about using castor on the eyelashes though! Keen to try this out.