2016 BUICK REGAL PREM 11, WHAT TYPE OF FUEL SHOULD BE USED

I have A 2016 Regal Premium II, This is out of the owners manual: If the vehicle has a 2.4L L4 engine, use regular unleaded gasoline meeting ASTM specification D4814 with a posted octane rating of 87 or higher. Do not use gasoline with an octane rating below 87, as it may cause engine damage and will lower fuel economy. If the vehicle has the 2.0L L4 turbo engine, use premium unleaded gasoline meeting ASTM specification D4814 with a posted octane rating of 91 or higher. Regular unleaded gasoline rated at 87 octane or higher can be used, but acceleration and fuel economy will be reduced

I used premium for about a year when I commuted 22m via highway back and forth everyday for work, however once I relocated and started taking the backroads, I figured I was better off with the reduced acceleration anyways and tried filling with regular. Performance did not take a huge hit and millege dropped about 2 mpg in the city. I was concerned about running into problems or a knocing sound that I was warned to watch out for by others online but its been running great since!