I would agree to use the octane the engine is designed for. I know of a exception to that. Those driving the Ford Taurus made around 2000 with the 3.0 V6 engine with automatic. It was designed for 87 octane. But it runs much better with improved gas mileage if you use 89 and some use 91 octane.

My wife bought a used 1999 Ranger pickup. I uses a similar 3.0 V6 engine and has a automatic transmission. It's always been sluggish in low gear and not much power in second. She tried a few gas additives like techron and later lucas. But when she went to 89 octane then it ran like a new engine. She also picked up little more mpg in the process.

None of that will work with the excellent designed engine like we have in the TCH.