Mercury 40HP Outboard Engine Oil & Filter Change

Engine oil in a Mercury outboard needs to be replaced regularly, at least after every 100 hours of use or once per year. The motor oil should be changed more often if you do a lot of wide-open throttle running. 

Watch the video above and follow the steps below to learn how to change the oil and filter on a Mercury 40HP four-stroke outboard. 

NOTE: The steps to change the oil on a Mercury 40HP are the same or similar for most makes and models of small-to-medium sized outboards.

Tools and Parts – Mercury 40 Outboard Oil Change

  • Ratchet
  • 18mm socket
  • Oil drain pan

Mercury 25W40 marine engine oil

Buy Mercury 40 fourstroke oil filter

How to Change Mercury 40HP Engine Oil & Oil Filter

Step 1. Remove the top cowling, then tilt the motor up as far as it’ll go and turn it so the drain bolt is facing downward.

Step 2. Place a drain pan under the outboard, and remove the 18mm drain bolt and its nylon washer from the driveshaft housing.

Mercury 40HP outboard oil drain bolt

Step 3. Wait for the old oil to drain out, and inspect the nylon washer while it’s draining. Replace the washer if it’s damaged 

NOTE: Replacing the washer is recommended after every couple of oil changes regardless of its condition to help ensure there are no leaks in the oil system.

Step 4. Reinstall the drain bolt and nylon washer, then clean away any excess oil from the driveshaft housing using contact/brake cleaner.

Mercury 40 HP outboard engine oil change

Step 5. Tilt the motor back to the vertical position.

Step 6. Put a rag or shop towel under the oil filter (located on the left side of the powerhead) to catch any oil that may spill out, and remove the filter with a filter wrench

Mercury 40 outboard engine oil filter

Step 7. Lightly oil the rubber O-ring on the oil filter and install it. Tighten the filter until it’s finger tight, then give it one more 360-degree turn.

Step 8. Remove the filler cap from the back of the powerhead, then fill the outboard with 3 quarts of Mercury 4-stroke marine engine oil and replace the filler cap.

Mercury 40 4-stroke engine oil change

Step 9. Run the outboard on idle for a minute to distribute the oil throughout the motor. Next, shut off the motor and let it sit for a minute so the oil can settle.

Step 10. Check the oil level on the dipstick, top it off if necessary and you’re done.

Ready to do a little more maintenance on your Mercury outboard engine? Watch the video above to see how to change the gearcase oil on a Mercury 40HP 4-stroke engine.  



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