What does the symbol 油 on Jiraiya's headband mean?
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.