Is this good or bad for a car?

Your car is designed to take a specific octane level when it comes to the gasoline you put in it. If the manufacturers specifications calls for an N87 and you're putting in N91m then you are probably firing your fuel off at a higher temperature. Although you may think you're doing your vehicle a favor by adding the better quality fuel, you are actually going to cause more wear and tear on it at a quicker rate. Also, you will void any type of drivetrain warranty or engine warranty by using the wrong types of fuel. The dealership mechanics will take samples of any damaged parts and test the chemical compounds of them as well. If it turns out that you have been using the wrong fuel, then you will be liable for the costly repairs to your engine. I would definitely recommend only using the fuel that your vehicle's manufacturer specifies. Doing otherwise will cost you a fortune in the long run.