Why is oil poured on the idol of Shani Dev

Astrological texts prescribe the practice of pouring mustard oil on the iron idol of Shani Dev to appease him. This is a form of anointing.

According to the Ananda Ramayana, when the influence of Shani Dev began on Hanuman, work was underway to build a bridge over the sea. There was always a fear that demons might harm the bridge. Hanuman was entrusted with the responsibility of protecting the bridge. Shani Dev, aware of Hanuman's strength and fame, explained his intention to the son of the wind by explaining the laws of planetary movements to a living being. Hanuman stated that he did not want to violate the laws of nature, but serving Rama was paramount for him. He implied that after Rama's work was accomplished, he would surrender his entire body to Shani Dev. However, Shani Dev refused Hanuman's request. As soon as he became formless and mounted Hanuman's body, Hanuman began colliding with huge mountains.  Whichever part of Shanidev's body he was sitting on, Mahabali Hanuman would hit that part against the hard mountain rocks. Shanidev was badly injured. Every part of his body was hurt. Shanidev apologized to Hanuman for his actions. Hanuman took a promise from Shanidev that he would never cause pain to any Ram devotee. After being assured, Ram devotee Anjaniputra graciously gave Shanidev mustard oil, which soothed his pain as soon as it was applied. Since then, to please Shanidev, he is anointed with bitter oil (mustard oil).

According to another belief, Saturn's metal is lead. It is also called naag dhatu in Sanskrit. Sindoor is made from this. It is also a poison. Its various uses are discussed in detail in Tantra Shastra. Iron is the metal of Mars. Oil is also called 'sneh'. It keeps iron safe. It does not let rust form on iron and if rust has formed, it cleans it. If Mars is strong, the ill effects of Saturn are eliminated. This is said to be a simple way to pacify Saturn. Offering oil means surrender.