During medieval age and earlier in Earth's history, oil was used as an incendiary weapon to burn animals, people, wooden fortifications and well anything that could burn.
It's speculated that one of the earliest incendiary weapons, greek fire, was based on oil.
I don't have significant knowledge in military weaponry and tactics but as I understand it oil was abandoned in modern weaponry and replaced by refined petroleum and other chemical products.
Napalm for example, (which has similar properties to greek fire) was based on gasoline and plastic polystyrene.
In my world, petroleum is extremely rare and only a few countries have access to this resource.
I know that petroleum is pretty useful for many things other than producing (and if it's rare you can just sell it) incendiary weapons but that is not my concern.
If a country has access to petroleum, why would they abandon it as an incendiary weapon?
What are the disadvantage or using oil?
In our modern society for example, why was oil abandoned as an incendiary weapon?