Once a year as a shock treatment or shock treatment and add to every fill up?

With the garbage gas, every tank.
I prefer mercury power tune for the shock treatment, two cans and it cleans more gunk out than seafoam.

per how many gallons?

probably about 5, I did an 82 johnson on my last boat when I bought it back in 08 but used seafoam. 2 cans in 5 gal. and run the heck out of it on the lake, you can do muffs but do it over the grass as you will turn concrete black and it won't come out very easy.

that engine was running really strong when I sold it to get a 96 fast strike 175. power tune or seafoam either will work then one can of seafoam every other fill up if you run a lot, it is also a preservative like sta bil.

The first time I cleaned my 2 cycle outboard it had compression readings of 103 psi across 4 cylinders. I cleaned it with Seafoam. One can of Seafoam, 1 gallon of gas and oil 50/1 mixed together. I ran the motor 15 minutes, let it set for 30 minutes and ran it another 15 minutes. The compression increased to 115 psi across all cylinders. That is not bad. I don’t run Seafoam day to day. I shock clean with Seafoam every couple of years now and regain about 5-10 psi.

never would I use seafoam in a outboard motor

Why not

He’ll say that it contains alcohol, then we can say, well use the marine version; because it contains zero alcohol.

I use the Marine when I use it. On every full tank fill up for me.

No over a period of time it will powder up in gas tand and screw up filters

How do you know this?

Seafoam is nothing but a mixture of kerosene, naphtha, and alcohol.

First I’ve heard of that one, but I’m not a mechanic. Is the mercury stuff and Yamaha ring free supposed to do the same thing?

I’ve used it in every outboard I’ve ever had and no issues