I just tried a tank of that Shell 91 octane super crap on a trip and my engine went insane!
I don't know what is in it, but my manually set up 258 would 'not' run over 65 mph except on a down hill, I even had to gear down on a couple long uphill runs, which is unheard of for my rig, even 3rd gear wouldn't pass 65 mph! (normally 3rd pulls fast up to at least 75 mph)
I also lost about 30% for gas mileage. It made for a long trip home too.
I finally got it low enough and put in Esso gas and magically my engine came back alive with it's normal tire chirp off a fast start and top end power restored.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's