Now that gas prices are so high, the differential in 87 to 93 isn't that much. I agree with what others say- pony up to get the best gas your car was designed to use. The lower octane is meant to be used temporarily and the knock sensor will decrease the timing. As these cars age it's even more important to use the proper gas.

My mother in law's 1995 525i was pinging on 87 and she was told to use higher octane given the higher mileage of her car and chance of detonation damage. Granted that car probably doesn't have knock sensors, but still....why keep the car in more or less a limp mode. If you want to save gas $ and don't car about performance, the diesels are probably more suited to you (with added benefit of the even longer longevity of those cars)