Some additives do what they claim but proper maintenance will eliminate the need for most of them except for maybe fuel cleaners.
Witnessed both Seafoam and Marvel Mystery oil unstick lifters....because the oil wasn't changed regularly. Same goes for stuck rings.
Seen both Lucas Transmission Fix and Dr Tranny Shudder Fixx (yes, 2 xx's) fix a slipping transmission because the fluid wasn't changed enough (or at all) or the vehicle was driven very hard and the clutches were worn.
Seen head gasket sealers work (NEVER use them on an engine you care about!) because the coolant wasn't changed regularly or the engine was overheated...because the coolant wasn't changed on a regular basis.
Seen Engine Restore curtail and eliminate oil consumption due to worn cylinders from infrequent oil changes. Also seen it work on engines with very high mileage that were properly cared for.
I've used seafoam in small engine equipment I acquired that were misfiring due to clogged carbs but still ran...kind of. Pour a hefty amount in the gas tank and the engine runs butter smooth 5-20 minutes later. 100% successful on a shredder, weed eater (free), and 2 rototillers (the Honda rototiller was free, the rear tine was dirt cheap). Saved me time and money rebuild carbs or buying a Chinnese knockoff and swapping it out.
29 years ago, had a buddy with a Toyota truck (high mileage) that would grind a bit in 2nd gear when cold. Told him to try some Duralube and see what happens. Fixed the problem. He still has that truck.
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