Naphtha is just part of the solvent system used to keep the active components of a fuel system cleaner in solution. There are several different types of naphtha, but all are just petroleum distillate cuts that contain linear and cyclic hydrocarbon chains containing 12 or fewer carbon atoms. While these short-chain hydrocarbons are good cleaners when used in concentrated form, they don't do much of anything when diluted into gasoline, WHICH ALREADY CONTAINS NAPHTHA.
Seafoam works well as a cleaner and carbon remover when it is used as it was originally intended. That is, to shoot the straight stuff into the carb until the engine stalls and then let the engine sit for a while, which allows it to soak into solid carbon deposits. Starting the engine then allows the carbon to flash off out the exhaust producing copious amounts of smoke. Seafoam is mostly naphtha along with a heavier distillate cut that provides some lubrication to protect 2 stroke engines. Running Seafoam diluted with a little gasoline from a remote tank (the Dunk procedure) also seems to work well. But adding Seafoam to a tank of gas at the recommended dilution would do little to improve the cleaning properties of gasoline. I think this is just a marketing ploy as is the recommendation to add Seafoam to crankcase oil, which makes no sense to me at all. We have enough problems with fuel dilution in outboards.
Ringfree and Mercury Kwikleen are the same product. They were made by a subsidiary of Chevron and contain a concentrated form of Techron (
polyetheramine or PEA) in a solvent mixture of naphtha and other distillates that are the same or similar to Seafoam. The solvent mixture is just a carrier and doesn't provide any significant cleaning action. PEO changes the structure of solid carbon combustion products as they form making them less likely to deposit on metal surfaces. When used at higher doses (shock treatments), Ringfree can remove some existing carbon deposits in cylinders and throughout the fuel system. You need to change the oil in a 4 stroke engine after doing a shock treatment, which is why Yamaha no longer recommends anybody except a certified dealer do the job. Most Techron formulations are now off patent, so other companies use them as additives in Top Tier gasolines. As long as you are using a Top Tier gasoline, you probably don't need to use Ringfree for every fillup. Yamaha recommends its use for insurance against poor quality gasoline. Since most Rec 90 fuels are unbranded, it is probably a good idea to add Ringfree most of the time. POAs are a proven technology that works very effectively.
Startron is an enigma to me. I once did a good bit of research trying to find out what it is and how it works. It contains naphtha and other petroleum distillates along with a proprietary "enzyme" that is supposed to prevent fuel oxidation (act as a stabilizer), prevent deposits from forming, and to remove existing engine deposits. My problem with this is that enzymes are proteins and all require some water or a hydrophilic solvent in order to work. I can't understand, from a chemistry perspective, how enzymes can work in pure gasoline. My research led me down a rabbit hole of chemistry gobbledygook. The technology seems to have originated from a Japanese company that talked about enzymes changing the structure of water and a bunch of crazy stuff that was either incomplete or just nuts. They may be using the term enzyme for catalyst, which them makes some sense. Regardless of my skepticism, many people swear that the stuff works.
With the possible exception of Startron, none of these additives contain anything that will stabilize fuel. For that, you need products that contain anti-oxidants and metal inactivators. Stabil is the oldest product sold for this purpose and as most people will tell you, it works as advertised. But there are other products with similar properties sold by engine manufacturers that all seem to work. Just be sure that the additive is a "fuel treatment" or "fuel conditioner" that is marketed to stabilize fuel for at least a year.
One final bit of information is that the "Plus" designation on Yamaha Ringfree and Fuel Stabilizer and Conditioner refers to a metal filmer additive that protects soft metals such as aluminum from corrosion. These were originally developed to protect fuel level sending units from ethanol-induced corrosion. They are added to protect outboard fuel system components such as the VST from corrosion. They prevent the dreaded white power from forming downstream from the main fuel filter and clogging up the VST filter.