This question is not re the use of qui(subject) vs que(object) in relative clauses.
It is the concept as the lesson stated of "If it refers to the whole part of the sentence, the whole idea, then it will be ce que/ce qui."
The examples in the lesson are pretty straightforward.
But does the grammar rule "If it refers to a noun (expressed before), then you will use que/qui...TRUMP the 'concept' guidance.
In the sentence,"the oil,which was supposed to burn for a day, burned for eight days. ** Note the commas please **. The 'which' clause is not really further describing the oil. It is not similar to "the oil which(that) I used". It is pertinent to the entire miraculous situation/idea . What was incredible was that the oil burned for eight days.. nothing about the OIL itself was incredible.
Even in writing this question, the thought process gets tangled up between grammar rules and context. And here the context seem to defy the grammar rules.