Oil is Ennai -which came from the words El +Nei = ELNEY==Ennai,
which is the extract of ellu i.e gingelly oil.
The oil or the abhishekam material is such that it should not corrode the vigraham-- murthy.
Moreover it should have property to carry micro quantities of the silai-eg Krishna silai) , that the out-coming oil ,ghee etc after abhishekam can be used as medicine which can cure chronic diseases.
for eg - Abhishekam oil from Guruvayur temple is believed to be cure for Rhumatism.