I have an old Mercury Thunderbolt 650, s/n 2311662. I would like to know what year it is and what is the correct gas/oil mixture. I bought the boat/motor/trailer for a few hundred bucks several years ago just to get the boat as the motor had a welded lower unit and I assumed it was trash. Instead the motor works very well and I have not touched it in many years. Now I think maybe I should do some maintenance. What are the best plugs to use these days on this old beast? I assume normal hypoid for the lower unit? Thanks..

Holy Crap that's old!
I assume 50:1 on oil although a little more couldn't hurt
Merc shows a Champion L19V

Change your impeller