As indicated above, short-term testing is of little value. However, in the long-term, mileage records can be very helpful to determine the usefulness of additives, from an economic point of view. I have used various additives, and combinations of additives, in the same vehicle (2001) over more than 100K miles. My conclusion is (and has been for over 10's of thousands of miles now), the additives I use do, in fact, pay for themselves, monetarily. I cannot attest to any component longevity advantage. That would require a fleet of like vehicles tracked over their lifetime with meticulous record keeping.

For a couple years now, I have been using Power Service (gray bottle) and Supertech (Walmart) TC-W3 two-cycle oil (formulated for liquid cooled engines, as opposed to hotter air cooled engines) at about 8 oz. each per tankful (20-25 gallons). I can see, feel, and hear a difference if I don't use either or both of the additives. The engine is more quiet, smoother, more responsive, and returns better mileage. Period. YMMV, but in my case, it's obvious to me.