What type of oil did they want to boil Horton in?

In Dr. Seuss's "Horton Hears a Who!", the characters want to boil Horton the elephant in "oil." Specifically, they mention "boiling him in oil" as a cruel punishment for his insistence on protecting the tiny Whos that live on a speck of dust. This highlights the story's themes of compassion and standing up for those who cannot speak for themselves.