As the previous post said, BE POSITIVE the octane rating is the same or MORE than what your tune is. A 91 tune running on 93 fuel is perfectly fine, 93 running on 91 will destroy your engine, fairly quickly too. As for the pollutants, almost all fuel stations will have ethanol mixed in, which then leads to moisture, and can also damage your engine. The less ethanol, the better. You can get products that can clean your fuel system from all the deposits and contaminants the ethanol has created through extended use, but a large percent of ethanol in the fuel should be avoided, vigorously. If you don't know, don't risk it unless you absolutely have to. As the previous poster stated as well, a larger brand or station obviously has something right, and is able to obtain a higher quality blend of fuel than a family run convenience station. As well, try and make it consistent, and try to always fill up at the same location because of the variability in the fuels if you can. At least try to stick with the same brand.
Trial and error is mostly the best way to go, or you could ask the operators what kind of blends they sell. Whichever seems to have the least affect on your vehicle, stick with it.