I don't know about that. I am no car expert, I am just going from what my dealer suggests. I don't get my gas from the dealer, and I don't get my oil changed there, so the dealer really has no motivation to lie to me. I simply asked their advice, and they said that an engine runs best when it is clean, and using high octane gasoline and synthetic oil gives you the products with the least chance of polluting your engine.
Higher octane is not necessarily good for your car. An engine is designed to function optimally (specifically, "when things that are supposed to go bang, do actually go bang"
) based on the appropriate mixture of gas (of a certain octane level) and air. If your car is designed to use 91 octane as many European cars are, then it will run better with that. If your car is designed to run on 87, using something higher than 87 doesn't do anything beneficial. If you use something lower, the timing of the aforementioned bangs may be a bit off, and you'll experience "knocking". I have been told that most modern cars will electronically adjust for a steady diet of whatever the new mixture is, but I figure why chance it.