There is a common belief that oil adds no value to a bait (in terms of attraction).
Oil is not water soluble and therefore cannot be detected by carp until it is in contact with the head and mouth i.e. as the bait is being "tasted" during consumption. Carp cannot "smell" the oil from afar, they simply don't have the tools. With this in mind, if you coat boilies with oil you are potentially locking in all the soluble attractors, preventing them from escaping into the water and being detected by Mr carp.
I don't know if I am in agreement with the theory above but I don't use a lot of oil anyway. Oil benefits bait in other ways (nutrition for example) but attraction is king for me.
A decent fish or liver hydroslate would always be my preference for coating boilies.