The OMC stern-drive marine engine has an upper and lower gear case that needs the proper amount of lubrication to keep the gears, bearings and shafts rotating without friction. Too much friction causes excessive heat and breaks down the oil's viscosity and lubricating qualities. Checking and refilling an OMC gear case is very different from the process used for conventional automobile engines. An OMC marine engine, like many other marine engine brands, cannot have any air pockets in the oil chamber, keeping air out of the system and attaining the correct oil level requires a two-part process.
Remove the boat from the water, if docked or moored. Trailer the boat to a suitable work location, such as a flat driveway surface. Position the motor so it sits vertically straight up and down. The motor cannot be trimmed or locked in an upright position. Place a drain pan under the lower unit to catch any overspill. Locate the lower unit drain plug, which will be at the bottom on the right side as you look at the motor from the rear. The drain plug will have a large slotted screw head. Refer to your service manual for its exact location.
Pour 2 quarts of oil into a large plastic container and screw on a hand pump adapter to the container filler neck. Use a large flat-head screwdriver to loosen and remove the drain plug, but quickly screw in the other end of the adapter fitting (on the hand pump) into the thread hole, immediately after removing the drain plug. Try not to lose too much oil in the process. Push the hand pump handle once to keep the pressure even so the oil does not back-drain down into the container.
Locate the upper vent plug on the lower unit. It will have a slotted screw just like the that of the lower drain plug. It will sit about 1 foot higher on the right side of the lower unit case, just below the cavitation plate. Consult your service manual if unsure of its location. Use a large flat-head screwdriver to remove the vent plug. If no oil dribbles from the vent plug hole, begin pumping the handle on the hand pump. When you see oil beginning to flow from the vent plug, pump the handle three more times and quickly turn the vent plug back into its hole.
Tighten the vent drain plug with the flat-head screwdriver. Unscrew the pump adapter from the lower drain plug and quickly screw in the lower drain plug. Tighten the lower drain plug with the screwdriver. You have now filled the lower unit with oil correctly, but you must check the upper gear case level. Use the screwdriver to remove the upper vent drain plug, and then quickly screw in the hand pump adapter into the thread hole, without losing too much oil.
Look on the top (flat part) of the upper gear case. You will see a plate with four bolts secured to the top case. In the middle of the plate you will see a large slotted screw, which measures 3/4 inch. This is the dipstick. Unscrew the dipstick and look at the marked line on the end of the dipstick rod. Wipe the dipstick with a rag and screw it in until it seats.
Unscrew the dipstick and check the level. If the oil measures under the line, pump the handle on the hand pump two times, then screw the dipstick back in. Do this slowly as many times as it takes to bring the oil level to the line on the dipstick. After reaching the proper level, insert and tighten the dipstick. Remove the pump adapter from the vent drain plug and quickly screw in the vent drain plug. Tighten the vent drain plug with the large flat-head screwdriver. Wipe down the lower unit of any oil and remove the drain pan.
