I would Never put 87 in a BumW as I'd never Have one.
The "cleaning effect" of premium over regular is an old wive's tale. The same brand of gas has same cleaners for each grade. If you so much as get regular from a Top Tier station instead of even Premium from Cheap Charlie's, it Will have better cleaning abilities. But, no more than TT's premium.
And, premium May afford Slight; 1 MPG; increase in economy and cooler running. You wont even notice the temperature change on a liquid-cooled engine. I put 104 UL in an air-cooled motorcycle that was speced for 87; I noticed absolutely no change in performance, starting, MPG. Only slightly cooler running. And, she has electronic spark advance. If I could manually advance it; I'd realize some difference.
Your mechanic is wrong or you heard him wrong. IDK, I wasn't there. Maybe he has stock in Chevron?