1.  Prepare to Drain: First, you need to ensure that your outboard is in a vertical and upright position. Then you will need to locate and identify both the upper and lower Fill and Drain Plugs. Place an oil drainage pan beneath the outboard where you will remove the bottom drain plug. Some oil will begin to trickle out so you need to make sure that the oil drain pan is positioned correctly to catch the draining oil.

2.  Drain the Gear Lube Oil:  Now remove the top drain plug; this will release the vacuum that exists and allow the oil to drain freely from the Mercury lower unit. Allow all of the oil to drain completely from the lower unit. This process can easily take between 5-10 minutes.

Once all the oil has been completely drained, the lower unit will have to be re-filled. There are various tools available in order to re-fill the oil, such as squeeze bottles and tubes. It seems that the easiest way for most people is to use the Mercury pump & fill tube.

3.  Add Gear Lube Oil: To refill the lower unit, insert the bottle or tube into the bottom drain hole and slowly squeeze the bottle or push the pump down to force the oil into the lower unit. We recommend original manufacturer marine oil for best performance and longivity.  Read more about brand name vs generic oils here.

Continue to add oil until the lubricant is flowing freely from the top hole and is free from any air bubbles. Keep the bottle or tube in the bottom pug and reinstall the top plug. Quickly remove the bottle from the bottom hole and install the drain plug. It is important to do this as quickly as you can to minimize the total amount of oil loss. Even though there is a slight vacuum there will be some loss of oil. 
Tighten both the lower and upper drain plugs and wipe off any oil that may have fallen on to your lower unit.

A marine specific lower unit gear lubricant should only be utilized in a Mercury outboard. The additive in such oil is normally of a higher water demulsability and has corrosion protection. Failure to use your manufacturer’s gear lube and oil can have devastating, costly results.

Always ensure your lower unit oil has the following properties in order to function correctly:

1. Excellent seal compatibility
2. Excellent resistance to water and moisture
3. Great protection of components from rust and corrosion in dry conditions and in moist conditions
4. Enhanced gear, bearing and seal cleanliness

Also, be sure to consult your outboard motor’s owner manual to verify the steps above are in agreement with the manufacturer’s recommendation for changing lower unit oil.

Happy Boating!!!