Octane
The higher octane should not change your mileage one way or the other theoretically. What the higher octane fights is preignition. It will allow the car to fully compress the gas before ignition which, if you were having problems with preignition, will make your car run smoother and give you better mileage. If your car is not preigniting the fuel, then there will be no difference.
Some station do put a better detergent package in the high octane gas, so it should keep the fuel system cleaner and thereby help the car to run better.
All stations are supposed to put atleast a minimum amount of detergent in their gasoline, but studies have show that there are many that don't meet the EPA requirement. Most of the time the big name companies monitor their stations to make sure that they are putting the companies additive package in the gas. At the cheaper stations you will be lucky to get the minimum amount which will not keep your engine clean.
Buying the gas additives, STP/Gumout/Prestone type, will usually at a minimum double the minimum detergent in your gas. I have looked at several of these products and found that some have more additives than others. STP, Gumout, and Techron were usually the better products. I couldn't find any detergent in Lucas.
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