I own two 6.2L GMC's. My old one, a 2011 6.2L Maxx, now has 169k miles on it. It ran on 87 octane gas most of it's life, until, on a lark, I started running 91 octane in it, and immediately noticed a definite uptick in performance. So, then I looked in the owners manual, LOL, and sure enough, 91 octane is recommended. My new one is a 2017 Denali Ultimate with the newer direct injection 6.2L. The owners manual on the new one recommends 93 octane for full performance and best fuel economy, but lesser octanes can be safely used. My daughters 2009 Pontiac G8 GT 6.0L also recommends premium, but will run fine on 87, but with a decrease in performance. I run 91, no ethanol, in both of my 6.2L's now, since 93 is not available in my hood, as far as I know.
My point is, you don't have to run high octane gas in the 6.2L, but if you really want them to snort, 91 is the way to go. BTW, I am told by a former Ford salesman, that this is a common phenomenon these days, where the manufacturers rate the horsepower and fuel economy using high octane gas, but the engines run fine on lesser gas, but with a decrease in performance. Still better than a certain car I own, which is supposed to burn 91 ONLY.
