Sorry, reading my last post, I left a word out.  I meant to say I run some "decarb" straight into the throttle bodies.  I'm sure SeaFoam would work just fine for that also but it's not in a spray can, I buy the Johnson/Evinrude stuff that comes in the spray can.  I use two cans, one can in each bank, spraying both cans at the same time going up and down the throats until cans are about 2/3 empty, then load it up with the rest until it dies.

Any brand decarb should work, I've just used the Johnson stuff for buches of years and I've never had a problem with carbon buildup.

I have a 88 115 Murcury that still runs like a champ. While in the shop last summer for a flywheel problem, the mechanic (an old timer) advised me to burn 87 and to use Seafoam every other tank to keep the carbon from building. I really believe that it has made a difference in performance.I made sure that I added seafoam to the last tank of the season as a stabilizer. It fired right up this spring.

I run 87 octane in my 200 HO etec.  The dealer said DO NOT run anything higher or use any additives.  I'll listen to them until someone more reputable says something different.

Etec's are a different animal.  With the piston temps they run, you definetly don't want to add anything.