87, 89 and 91: Which fuel grade?

- Wanted my car to run longer, so even though my car manual suggested to use 87 grade fuel, I had used 89 for 3 years. On one fine day, decided to search more on the net and found that the best way to go forward is to use 87 grade instead as suggested. I have learned that using higher grade fuel is not going to benefit the engine because of the way it's designed. 

So which fuel grade do you use? Do you follow the car manual? Do you think higher grade fuel helps the engine? Do share your comments below.