According to https://www.tastesofhistory.co.uk/post/dispelling-some-myths-medi%C3%A6val-murder-holes

There are only a very small number of scattered references to the use of hot oil. It was prohibitively expensive, not often available in large enough quantities to be effective, would have been difficult to heat, problematic to transport around the parapets, and could have been a fire risk in itself.

There are far more accounts of boiling water, molten lead, and even heated sand being used.

And molten lead was easier to handle on the parapets? I wonder.