I used fuel system cleaners and oil system cleaners before oil change on my BMW E39 never had any problems. Drove it over 300.000 and engine looked like new, no leaks no knocking, run like new.
It does not matter if you use purest fuel in world it leaves trails when it burns that is why you need system cleaner. And for oil is same. I used this great product from Liqui Molly...you put it in to engine before you change oil. let it run 20 min and than drain it. It cleans all that burning traces and thick oil mud looking thing from engine. It was dirty every time...and i used always full synthetic quality oil in that engine.
But i never tried Octane boosters, i use 95 or 98 when i find it. Few times i tried 100 Octane Fuel but did not see any difference!