Shell v power 91 vs Petro Canada 91?

Why dont you ask the manufacturer????? Anyways its the octane that matters not what is put in the mixture of the fuel. 87 octane will detonate under less pressure then 91 octane. Therefore if you were to put 87 in it that would make a difference because it would detonate before it hit full compression so the spark plug wouldn't ignite the fuel when it is suppose to. This is because you probably own a car with high compression something above 10:1 probably. So in conclusion one companies 91 octane should preform the same as another companies 91 octane.