Transmission on 1999 Merc Mag 350
 

I was wondering if you can or need the change the Transmission fluid on these engines?...if so, how is done and what fluid to use ..thanks

There is no "transmission" per se in your boat. If you're referring to the mechanism that allows you to shift to and from forward, neutral and reverse, that's your stern drive and it uses Gear Lube. Generally speaking you change it at least once per season by draining the drive. You then refill it from the drain hole / plug until it comes out the vent hole / plug above. And although this may sound backwards, it's critically important that you do it that way.

+1 to the above. I run the engine on the hose for a bit in gear (careful of the spinning prop) to get the gear oil warm.
It takes quite awhile to drain all of the oil.
Some folks recommend running an extra 1/2 - 1 quart of oil through the drive to kind of flush out some of the old oil.
Changing the gear oil every 100 hours or once a year is the recommendation I remember.
You can get a pump for the 1 quart bottles of oil for under $20 (threads into the lower hole. Highly recommend this.)
I use Mercruiser full synthetic gear oil. Make sure you get two new o-ring gaskets for the lower and upper plugs you will be removing and replacing.

The lower plug is on the "nose" of your lower unit. The upper plug is near the top of the drive on the RH side.
You will need a fair-sized flat blade screwdriver for the removal and replacement.
The drives hold under 3 quarts so you won't need a large drain pan.
Plan on an hour with about 1/2 the time letting the drive drain.
Good luck.

..or if you forget to do it for 10 years, it wont change much on a stock setup like this. Change it tho if you dont know when it was changed last.

Ok thanks...I wasn't sure if there was a mechanism between the engine and lower units