Regarding the cleanliness of different octanes of fuels, you have to go back to where fuels come from before they get to the gas station. My city has about 300,000 people and there are two fuel depots. Here's what happens. A Shell truck will be sitting in line behind a Chevron truck at a Marathon fuel depot to pick up gas. The Shell truck will have Shell's proprietary cleaning agents, or additives, placed into the truck's tank before the gas is filled, or the agents are directly injected somehow into the fuel as it is going into the truck. All fuel coming from the large storage tanks at the depot is the same, other than their octane rating. Different octane gas sits in different tanks, just like at the station. Anyway, the Chevron truck will pull up and be loaded with identical fuel as the Shell truck, but with Chevron's proprietary cleaning and treatment agents. Generic fuel stations are typically cheaper because distribution trucks fill up at the local depots where Chevron and Shell fill up, but they don't put in special proprietary additives. So the gas is cheaper to bring to market. However, the fuel will over time leave deposits, regardless of octane rating, that name-brand gas will not.
So here's what I'm getting at. Some companies may put more additives in their higher octane fuels, but simply having a higher octane does not mean it will be cleaner. I guess saying the fuel is cleaner is a misnomer anyway. It's all the same fuel with different additives so it's what's left behind that defines the cleanliness of the fuel. I highly recommend that you buy 87 octane gas from a name brand station. It will probably have a better overall value than 93 octane gas from a no-name station with cheap gas. Personally, I prefer Chevron/Texaco because I've always bought Techron injector cleaner since the '80s and it's great stuff. Chevron/Texaco have that in their gas. I also like Shell if I can't find the others. Marathon is supposed to be good, too. If you have Sunoco stations where you live, I hear those are very good.
Now to the tuning part. You have to find "tuners". Sometimes you can find computers that are pre-programmed. This what being call "having a chip in the car" or "the car is chipped". You can take the car to a shop with a dyno and they will manually tune your computer, or an aftermarket computer installed in your car, to run on 93 octane gas. In most cases the engine will have slightly higher horsepower due to a slightly higher compression. Remember, higher octane gas has a more controlled burn and can be controlled more easily to detonate at a relatively precise moment. I can't explain in any detail, but higher octane also means higher compression is required to detonate. Auto engineers who develop fuel management schemes have probably the most important job in making an engine run seamlessly. Lots of work goes into the development, but any fuel management problems will make the engines problematic. Take for instance fuel management on the DISI Mazdaspeed 3. No offense to any owners of MS3s, but those engines have quirky, and in my opinion, suspect fuel management programs. But those engines must be tough to manage anyway so I'm not knocking the engineers too hard.
