I own a 2001 Audi TT 180 Quattro and the owners manual says the car requires 91 octane or higher. Since gas prices are getting very high I was wondering if I will damage my engine in any way by using 89 or even 87? Since I use this car to commute on the highway 50 miles each way I'm ok with a slight loss of power if I will save money at the pump. The car in 100% stock. Any thoughts will be appreciated! Thanks!