If a manufacturer states Premium gas "recommended" that is marketing code for "IT REQUIRES 91" octane gas.
There is not a car salesman alive that will tell a customer NO, it needs 91 or don't buy it when a customer asks the $69 question: "87 will be OK, right?"
wrong. Recommended, in marketing speak, means put it in if you want to match our official ratings for power, fuel economy, etc. Required means just that. Audis or MINIs "require" premium gas - DO NOT PUT IN REGULAR in those cars. Cars that say recommended, such as Saab, Volvo, or some other turbo cars, which means you can put it in but it is not required. This usually is the case if the engine is tuned to run on 89 octane or thereabouts - 91 gives it better everything, but it can retard timing back down to 87 safely.
That being said, I always tell my customers that although it is only recommended, the benefits of paying extra at the pump for the 91 will far outweigh the little extra you pay for the gas [see my post above]