can any one direct me to a service manual, or info on how to do this? this was my dad's boat/ motor before he passed away. he always took immaculate care of things so I've no doubt it's up to date, but better safe than sorry. 

 

i've watched a few videos, none specifically for that motor tho... thanks a lot. 

All lower units should be the same as far as changing the lower unit oil. There are two screws going into the side of it and the procedure is as follows:

 

1. Trim or tilt motor all the way down and place a drain pan under lower unit.

2. Remove bottom screw, then top screw.

3. Let drain.

4. Check seals or gaskets on screws. Mercury uses a felt washer. I don't know about Mercury's that old but I would imagine it's the same. These usually need to be replaced at every change. Scrape any of the old washers off the lower unit before re-installing screws.

5. Start filling lower unit from bottom hole and fill until oil comes out the top hole.

6. Replace top screw before removing fill nozzle. 

7. Remove fill and replace bottom screw.

8(optional) remove top screw again and slowly continue to fill the little that was lost when replacing the bottom screw.

9. Clean up and you're good to go!

 

I'd recommend wearing latex gloves while doing this since lower unit oil has a very potent smell to it and does not wash off easily. Changing it should be done probably every 6 months or so. Be sure to look at the old oil as it's coming out. If it's go a milky whitish tint to it it means it has been exposed to water, which requires immediate attention. Usually it's caused from the seals on the screws, but it could also be from the rear seal. Pretty easy job overall though. It should take 15 minutes up to about half an hour at the most. 

That lower unit would take quicksilver 80W-90. When you remove that lower screw do it slowly. If you get water before oil you need to have the lower unit seals replaced. I usually replace my lower unitl lube twice a season. I have never changed a lower unit that didnt have somewhat milky discolored lube. It will always be  a little streaky and lighter in color than it was when you put it in. But if its like a milky yellow color and doesnt look anything like it did when it went in (and stinks badly), its time for a pressure test becasue you've got a bad seal.

 

Super easy to do I would recommend buying a pump from walmart made especially for this.