Holy Oil

What is holy oil? 
The primary purpose of anointing with the holy anointing oil was to sanctify, to set the anointed person-or object apart as "holy" (Exodus 30:29)
Why is olive oil used?
Anciently, oil pressed from olives was considered the cleanest, clearest, brightest-burning, longest- lasting of all animal and vegetable oils. It was also purest of oils and was thus appropriate for holy anointing.