I would suspect that the 3L NA engines should be able to run on regular? I own a BMW E39 with a M54 engine, and have been running it on regular for over 100K km since I bought it, no fuel, black plugs, or any other problems associated with it. It will not perform at its peak power on regular fuel, and I can feel the timing retard at the mid to top end, on regular gas, but it does have Dual VANOS, so it will adjust. How often does one punch the car to run at it's peak power? Yes I will put 91 V power with no Ethanol on days I want to experience the rush, but on day to day, when the car runs below 3K RPMs, works just fine for me.
But there are folks who feel need to feel the prestige, and pay extra when they don't need too. Yes BMW recommends premium, but I suspect it would not be necessary, the car will adjust. On the Turbo Bimmers, Premium all the way!