My Saab can deal with 87, but it boosts higher and feels quicker on 94. The Volvo with the high compression ratio and the turbo needs 94 or in a pinch 91 to prevent knocking. I prefer to use 94 mainly due to the no ethanol content and hopefully slow the deterioration of the fuel lines that ethanol causes.

my 95 corolla i had b4, i used to run 87 but it knocks, tried 91 and higher and it quiet the car right down... and they said no difference? that is bs... my Sentra Spec V had noticeable difference in power even just between 91 and 94 octane... i am not gonna risk my Lexus when it recommends premium only...

It's as simple as the compression of your engine and conditions under which you drive, higher compression engines require higher octane gasoline. It's not rocket science, don't bother paying for 91 if you don't need it. The owner's manual for my S10 states that 87 is fine, my old Jetta had a label on the gas cap that said to use 91 octane ONLY.

I only put 94 in my Honda lawn mower

so conclusion of this thread is that all grades are about the same quality?

Technically yes, octane rating is resistance to knock so higher octane means less chance of knock. The only time increasing octane will help the vehicle is if it's knocking with the lower octane. But increasing it further won't do anything more for the car, you're wasting money by adding protection that wouldn't be needed.

Here's an example, if you have a car that weighs 2000lbs and you try to lift it with an 1800lb jack, it's likely to fail (knock). If you have a jack rated for 2200lbs it will lift it no problem. So there's no point buying a 2500lb jack or a 3000lb jack because you're just adding to a margin that isn't needed.

My opinion,
1- could be those with the older vehicles see a difference due to carbon build-up and/or blow-by (since the vehicles are older, more wear on the engine) creates hot spots in the combustion chamber that cause the engine to not run properly (pre-ignite or knock, you don't necessarily hear or feel it) and the higher octane fuel "band-aids" the situation.
2- In a newer healthy engine, higher octane will make you car run differently, loose power and may lead to deposit build up in the engine due to the unburned higher octane fuel your engine/ecu isn't designed to be working with
3-I encourage everyone to try different fuel brands, check your current consumption, then try a different brand same octane, check your consumption again, repeat with another brand. You'd be surprised how it can affect your vehicle.

4- It's your vehicle, feel free to fill it with whatever you want, whatever makes you happy