I recently bought a used 95 Acura Type S, and in the owner's manual, it says to only use premium gasoline of 91 octane or higher. Is this merely a recommendation, or can I damage my engine by using basic unleaded?

the different octane in gas causes the engine to fire at a different points in time.... so by not using what is recommend for your car in the long term could have a negative effect on your car. since the gas firing at different may actually hurt your cars performance....