Re: 75hp 1975 lower unit oil check/change
On your engine: Up above the cavitation plate there is the vent screw. Below the gearcase torpedo, on the side of the skeg, at an angle, is the drain screw. Both these screws are 3/8 X 16 with a large slotted head about 9/16 to 5/8 diameter.
On the side, toward the front of the gearcase torpedo, is a smaller recessed screw. This is the shift pivot and should never be removed unless you are disassembling the lower unit.
Since oil floats on water, checking the lower unit lube through the vent screw will not give a true indication of condition.
To check, you remove the bottom drain screw and loosen the upper vent, allowing only enough air in to let the oil dribble out. If free water comes out first, then for sure you have a leak. If milky oil comes out then there is water in the oil too. Milky means just that: Looks like cafe au lait. If clear or semiclear, top off the oil and run the engine.
To refill after draining oil: With both screws out and the engine vertical, fill with oil from the bottom (drain) screw hole until oil comes out the vent. A cheap hand pump available at auto stores helps here. Replace the vent screw and tighten snugly. Now remove the pump nozzle and quickly insert the drain screw. Wait about a half hour to let air rise then repeat the process without draining to top off the oil. Tighten both screws really well. Use a large screwdriver.
Note that depending upon condition, you may or may not need to replace the screw gaskets. Note also that frequently, the gaskets will stay stuck in the recess for the screw head. If this happens, don't think it is lost and add another on top.