I read page 298 of the owner's Manual...only if engine knocking occurs can you not use 87 octane...then 91 is suggested. Yes, performance/economy may not be as good with 87, so it says, but don't see it mentions a warranty problem.The inside of the gas cap door recommends Premium. The Owners Manual threatens warranty problems if you don't use premium. Buick Service gives a long political non-answer.
What do you think?
Never noticed knocking...been using 87 octane for 8,900 miles, in pretty close to sea level Texas.