On the subject of additives I did once use a fuel additive that really demonstrably worked. And worked fast too. Nearly 40 years ago I had a Ford Econoline van, the type with the engine between the front seats. I loved that van. Anyway, I was driving in Seattle on 520, leaving the city, and the lower radiator hose blew off. There was nowhere to pull over so I drove to the first exit, which was pretty close, pulled into the gas station there, and immediately shut off the engine. The engine stopped turning immediately too. I had the van towed home. A few days later I decided to see if it would even turn over. It did, and wanted to start, but couldn't quite. It also turned over faster than normal, meaning low compression. A couple days later I was in a NAPA auto parts store buying something for my VW Bug and related my van story to the counter man. He said it might just be coked up exhaust valves. He talked me into buying a can of "Valve Tune". The directions said to pour 1/2 can into the oil and the other 1/2 can down the carb while starting the engine. So I poured the stuff into the oil and then tried starting the engine. It would almost start. So I cranked the engine, poured a little down the carb at the same time, and the engine almost started. I let the thing sit a few minutes and tried again. This time the engine kinda started so I continued pouring the Valve Tune into the carb in a slow stream. Now the engine stayed running if I pumped the gas pedal just right. It ran real rough, like it was running on 3 cylinders. I continued pouring Valve Tune into the carb, tons of white smoke poured out the exhaust pipe, and the engine went from really rough running to smoothing out. By the end of the can the engine was running pretty well. I took the van for a drive around the block a few times and it started running well enough to risk a freeway run. I drove that van for a few more years and sold it, still running well. Since then I have not overheated an engine within an inch of its life and so have never needed Valve Tune again. But if I ever do I hope NAPA still sells the stuff!
Eric