Well, I can only talk for Australia and our refineries (what is left of them), but, in my state much of our gas/petrol comes from Singapore...

We basically have only Two types of Unleaded gas (our octane ratings are in RON)
91 Octane which is the cheapest per litre.
95 Octane about 10% more in price.
98 Octane ( Shell V Power) more again.

The high octane fuels here 95 and above are a clear Yellow in colour... where the 91's look more cloudy and a clear off white in colour.

I don't know if this is true or not but the Higher Octanes appears to be more refined and cleaner looking...I know sounds stupid, but someone will correct me.

My sister has a Kia Cerato, was running the 91, which the book says, I told her to try the Premium 95, it costs a little more, but she gets better performance and mileage, so IMO it is worth the extra cost.

ASh

 

unfortunately the only way would be to monitor the cars reaction, plus all the fuels are mixing with them, some gas station may have a leaky storage tank and so there may be water in there, tanks build grime and may pass it up, or the gas station could just be conning you and pumping 87 or 89 in anyway. In jersey we cant even pump our own gas technically (seriously) so we rarely can "see" it. I dunno if its rumor or fact but I've always heard that the mid-grade fuel (between regular and premium, most stations have different names) is simply a mix of regular and premium. That could be 100% wrong, if someone knows it would be good to hear. I know my evo can tell low octane, and I know all of mazda's turbo 2.3L motors have low_octane sensing abilities as there's a switch flag in the PCM for it which restricts throttle plate opening to like 30% or something, forget the specifics since class unfortunately. It's an available PID on the IDS/VCM PCM software system at the dealer, dunno anything about aftermarket. I should check if the 8 has a low_octane flag but i dont think it does.

kevin.