"Top Tier" fuels have to under go testing and certification to maintain that distinction. Now, I'm not saying they can't play with the mix and provide a beefed up fuel for the test, but testing is a requirement.
Where I live we get 87, 89, and 93 octane fuels. For my supercharged cars I use only Shell V-Power 93. My buddy works for an oil company and has told me that it is in fact all the same fuel with just different additive packages added at the time of shipping, and that's fine, but I like knowing at least Shell is tested to keep that Top Tier distinction.
In my Z, I just use the cheapest 93 octane I find. It's not a supercharged car so it's not as critical. Runs fine so far and I'm getting 23 MPG with a 70/30 mix of city/hwy driving.
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