I'm an old mechanic by trade in which im skeptical about anything like seafoam and such, i do run quickleen and quickcare in my 1995 merc. I have heard that doing what i did cleans stuff out ......seeing is believing and curiosity got the best if me...So i had a can of seafoam laying around that someone gave me and had nothing better to do so i gave it a whirl. Motor use to idle at 1,000 rpm on muffs and when done was idling at 1300-1400 rpm on muff's....never saw no black stuff come out or nothing.......the way we use to decarbon motors of old was to dump water down the intake on a running motor and the steam effect would knock some of the carbon loose in the cylinders....im not a true believer in additives but sometimes you just have to try stuff.
Now for fresh gas, when i lived in Florida i work for a City and when we got a load of gas i would talk to the driver sometimes.....found out that before gas is put in the tanker to be delivered they check octane of the gas, if octane is low they purge the gas with methane to get the octane up so that it can leave the port and be delivered....so now how long does that methane stay in that gas and how long has that gas been sitting in storage tank before it gets put into an tanker and delivered...now we have ethanol added to gas to help with octane and we all know ethanol is alcohol and evaporates pretty quickly, so after several transfer's from one container to another container how much of that ethanol dissipates out of the gas and just how fresh is that gas...