Question about the phrase "fought fire with oil" as used in the film "Spartacus"
It's a play on words of fight fire with fire by using a word (oil) from another idiom, namely
to grease or oil someone's palm
to bribe someone
See The Free Dictionary, which gives an example of both 'grease' and 'oil' in the same statement:
If you want to get something done around here, you have to grease someone's palm. I'd never oil a police officer's palm. That's illegal.