Re: Older OMC bottom end ?
no need to use synthetic, 80-90 outboard gearcase oil is all that is needed. walmart sells Super Tech Brand which is fine. 1984 is not old for an outboard.
Changing lower unit oil
There are 2 straight slot screw on the starboard side of the lower unit. Top is vent plug, bottom, drain and fill plug. ( do not touch Phillips head screw).
Remove the bottom plug, put container under lower unit, now remove the vent plug, while the oil is draining, replace the seals on the plugs part # 311598, buy several of these, ( they are a one time use item)
Walmart and many places sell a little hand pump. To use to fill the unit, you insert the tube into the bottom hole, and pump till oil reaches the vent hole. Install the vent screw, then remove hose, install the drain-fill screw, clean up mess.
The screws can be hard to get out, do not mess up the head. You may need to employ an impact screw driver to break them loose.
Can I use Synthetic Gear Lubricant made by Sierra? Says it replaces Volvo Synthetic Gear Lube but doesn't say anything about what viscosity it is. Thanks in advance.