Jiraiya is shown wearing a headband with 油【あぶら】 drawn onto it. I know that 油 is the symbol for abura, and that it means "oil".
But how is it related to him? Does it have anything to do with the fact that he is a sage from Mount Myoboku?
The kanji for Oil 油 【あぶら】(Abura) is the symbol of Myobokuzan. Jiraiya wearing it on his forehead protector represents his affiliation to it. The symbol probably comes from their special sacred oil, which allows people to feel natural energy more easily, and evaporates outside Myobokuzan's atmosphere.