Thanks for the advice I'm sticking to premium.
Forgot that while I wanted to make it clear why premium is important to use the flip side is it is only important, can only provide the intended benefit, provided the engine is designed for premium.
I didn't stress this in my previous post. Let me do so now.
There is little benefit in running 91 in an engine designed to burn 87 or 89.
With a couple of possible exceptions.
One exception could be if you track the car, a higher octane provides a bit extra margin against detonation so the engine controller won't have to retard ignition. Engines as they are used can have their octane requirement increase a point or two and this and what the engine experiences on the track could require a higher octane.
Another exception might be if you were heavily loading the car with passengers and their luggage or towing a trailer. I have no observations though to back this up but just knowing the engine is going to be working harder and could encounter operating conditions in which the engine cylinder filling exceeds what it has ever before the next higher grade of octane could be called for. In this case the higher octane gasoline provides a bit extra margin.
But except for the above exceptions if the engine is designed to run with 87 or 89 then using a premium grade of gasoline is ill advised. It wastes money mainly.
Be sure you buy gasoline from a busy station. Premium gasoline, especially, goes stale and quicker than the lower octanes of gasoline and if you fill up you want the freshest gasoline for your money.