Heppel oil was an extremely powerful aphrodisiac. It is secreted from the Heppel gland, a special (perhaps biomechanically engineered) gland that was usually implanted in Dellos concubines. According to Sikozu Shanu, this substance was only produced by such a gland.
The oil was produced by the gland seemingly at will and exited the body through skin pores like sweat It induced overwhelming lust when inhaled, rendering even the most determined member of the opposite sex powerless to resist any command given by the producer of the oil.
The means of producing and implanting the Heppel gland are unknown, although it was known to shorten the life of the person it was implanted in, a side effect which made it illegal. It could only be defeated by the previous inhalation of something which affected the brain and prevented sexual stimulation, such as laka guts. ("What Was Lost - Resurrection")
Commandant Mele-On Grayza secreted Heppel oil, which became very useful to her in her ambitious climb up through the Peacekeeper ranks.