How to Change Mercury 20HP Outboard Oil

Changing the oil on a Mercury 20 outboard is recommended after every 100 hours of service. Grab a couple of tools, some oil and the filter, and you can knock it out in minutes.

Watch the video above and follow the steps below to change the oil in a Mercury 20HP 4-stroke outboard motor.

Tools and Supplies - Mercury 20HP Outboard Oil Change

• 16mm socket
• 3/8th ratchet
• 65mm oil filter wrench

• Torque wrench
• Mercury 20 oil filter
• Mercury 4-stroke 10W-30 oil (2 quarts)

Mercury 20 Outboard Oil Change

Step 1. Lift the motor by pulling up the lower unit from the bottom to tilt the motor up, while pulling down on the lever on the right hand (as viewed from the rear) side of the motor. This lever will prop the motor in place. 

Step 2. Place an oil drain pan underneath the motor, then remove the drain bolt to start draining the oil.  

Step 3. Replace the drain bolt and return the motor to its vertical position. Leave it there for a few minutes to allow the last of the old oil to get to the foot of the motor. Lift the motor again, remove the drain bolt, and drain the last of the oil.  

Step 4. Replace the drain bolt after all the oil is drained. Use a torque wrench to tighten the bolt to 17 foot-pounds.  

Step 5. Release the lever again and lower the motor. Next, remove the cowl by lifting up on the latch at the back of the motor and then lifting the cowl off. 

Step 6. Stuff a rag underneath the oil filter to keep oil from running out of it and down into the engine cover when you remove it, then use a 65mm oil filter wrench to remove the oil filter.  

NOTE: There's a gasket on the oil filter. Make sure it comes off with the filter and isn't stuck on the engine. Otherwise when the new filter is installed, this will cause it to leak.  

Step 7. Use your finger to apply a thin coat of oil on the gasket on the back of the new oil filter. 

NOTE: This step makes it easier to remove the filter when it's time to replace it. 

Step 8. Replace the oil filter. Thread it on by hand until the gasket contacts the engine. Then, use your 65mm oil filter wrench to tighten it another ¾ turn. 

Step 9. Remove the oil fill cap at the rear of the engine, and fill the engine with motor oil. 

PRO TIP: The Mercury 20HP outboard engine takes 1.1 quarts of oil, so add 1 quart, replace the cap, and run the engine for a couple of minutes. When you run it, make sure to have water circulating through the engine as you would when you flush the engine out after every use so that it doesn't overheat. 

Step 10. Remove the dipstick on the right side of the engine, wipe it off, and push it all the way in until it bottoms out. Then, remove the dipstick again and check the oil level. 

NOTE: With 1 quart, our oil level was low. The three holes in the dipstick represent low, medium, and high. Mercury specifies the oil level should be around the middle hole.

Step 11. Add a little oil if the level is low and check it again. 

NOTE: Since we already added 1 quart, this should require about .1 quarts to get the level right in the middle range.

Step 12. Replace the dipstick once the oil level is set properly, then put the cowling back on and you're done.