This for a 2002 25 hp Johnson 2 stroke ,what say you ,synthetic or conventional mixing oil and feel free to be specific with what you use

I've had no issues with regular old Quicksilver or Mercury 2 cycle oil in the black jug. Ran one or the other for 30 years and never had engine problems. Changed plugs every 3 to 5 years and they were never fouled.

have you contacted Johnson?
Or a good OMC mechanic?
Or a Johnson dealer?

All you'll get here is opinion and very little fact

No offense furbarguy ,asking the dealer is like asking if you should have your vehicle service by them or someone else, I generally think of dealers as snake oil salesmen

Amsoil 2 Stroke is more like a wax than an oil. It makes the smoke issue tolerable and is not a petroleum type of pollution. There is a bit of a sheen (like dish soap rather than petroleum) produced at idle. Expensive but in a 25 HP I would guess you would be very pleased. I used it in a 1997 90 Yamaha 2 stroke in Alaska for years.

I have a 48 hp Evinrude that I bought new 32 years ago. In all that time I've run only OMC conventional oil and either Stabil or OMC 2+4 conditioner (which used to have an MSDS identical to Stabil, even made by Gold Eagle). This spring it went in the shop for the first time for carburetor rebuild to fix a 32-year accumulation of fine crud. In the process of troubleshooting what caused it to drop a cylinder the compression was checked; good in both cylinders. I see no reason to change oil.

Not just with outboards, but with other engines in general, I've always figured that the engineers who designed something know best, and that most products marketed by the manufacturer of an engine are well-suited to that brand of engine. I've had similar results as above using Stihl oil in my 41-year old Stih saw and an Echo trimmer that's about 30 years old.

Back in the 70s I had a friend who generally ignored manufacturer recommendations, as well as the outboard mechanic at work, who was OMC certified. He put some hot rod aftermarket prop on his almost-new 135 Evinrude and promptly blew it up. It could have also been because he ignored break-in recommendations and ran top end all day first day in the water.

I have a 22 yr old Merc 25. I ran it for years with the cheap oil but it smelled so bad I switched. I went to Amsoil 100/1 . It was much better but one day I got a pint of Merc TCW3 and used it and thought it smelled even less so I stayed with that the last 5 yrs. i run it 2 oz. per gallon and that is somewhat lean at 64/1. I read in a merc owners manual that it is safe to run 100/1 if used a lot i warmer weather . I did 100/1 on my old Merc and never had a problem with the engine. I figured the price of Merc TCW3. is cheap so I stick with it.

How long you been running outboards ? Been running them over 40 years myself and using the oils I recommended and never had a powerhead prematurely wear out. I wish I knew the hours that I had put on 2 of them I had.

Ihave an evinrude 15 hp, made in 1955, the best oil for it and all 2 stroke evinrudes is evinrude XD50

To the OP's question, I've been running mostly OMC for about the last 50 years. No matter the brand, use the mfg recommendations as to typs and weight. Never had a seized piston. Most of my engines are 2-stroke OMC's from 1968-1989. I like the synthetic Johnson Evinrude oil as it seems less smokey and am switching over, but am burning through my supply of dino oil first.