Mercury 40HP 2-Stroke Lower Unit Oil Change

Remembering to change the engine oil on your Mercury outboard is easy, but it's also easy to forget that the lower unit needs its own individual oil change. 

Watch the video above and follow the steps below to change the gearcase oil in a Mercury 40HP 2-stroke motor. 

NOTE: We changed the lower unit oil on a Mercury 40HP 2-stroke engine, but the process is the same or similar for most small-to-midsize outboard motors.

Tools and Parts – Mercury 40HP 2-Stroke Gearcase Lube Change

• Drain pan
• Flat-blade screwdriver
• Mercury gear lube pump
• Gearcase lube

Mercury 40HP 2-Stroke Lower Unit Oil Drain

Step 1. Remove the oil vent screw and washer from the right side of the outboard, just above the anti-cavitation plate.

Step 2. Put a drain pan under the lower unit, then remove the oil drain/fill screw and washer from the right side of the outboard at the bottom of the gearcase.

Step 3. Let the old oil drain out of the gearcase, and check the washers for wear or damage while you wait. Replace the washers if necessary.

Pressure Testing Mercury 40HP 2-Stroke Lower Unit

PRO TIP: With the gearcase drained, now is the ideal time to pressure test the lower unit to make sure the seals are good. Attach a pressure tester to the lower unit, then pressurize the gearcase to 14 PSI and let it sit for a minute. If the pressure remains constant, the seals are good.

Mercury 40HP 2-Stroke Gearcase Oil Change

Step 4. Attach a quart pump to a quart bottle of marine lower unit gear lube.

Step 5. Pump gear lube into the lower unit until it begins to flow from the vent screw hole. Next, reinstall the vent screw and washer.

Step 6. Disconnect the quart pump from the lower unit, then reinstall the drain/fill screw and washer.

Step 7. Clean the lower unit with contact/brake cleaner to remove any excess gear lube and you’re done. 

Looking to change out the gearcase lube in a 4-stroke outboard instead of a 2-stroke? Watch the video below to see how to change the lower unit oil on a Mercury 40HP 4-stroke outboard.