Whether you are boating on freshwater or saltwater in Georgia, South Carolina, or North Carolina, the most lower portion of an outboard engine plays a critical role in the vessel’s operation. Maintenance must be routine to the area where power is transferred from the top called the powerhead to the propeller sticking out the backside of the most lower section. Gear oil reduces friction on the lower unit, thus reducing the likelihood of failure. Here is why changing lower unit oil is essential. 

What Does Lower Unit Oil Do

First, before explaining the purpose of the oil, it is vital to understand the function of an outboard lower unit. The lower unit transfers the energy generated by the engine, otherwise known as a powerhead, to the propeller. When transferred, the propeller spins in a clockwise or counter-clockwise motion moving the vessel forwards or backward. 

A rod mounted to the base of the motor spins at a rate determined by the position of the throttle. The shaft connects to the lower unit by a series of gears called pinion, reverse, or forward gear. Energy from the powerhead spins the forward or reverse gear, thus turning the propeller depending on the direction the operator pushes the throttle. 

Gear oil, otherwise called lower unit oil, plays a fundamental role in the process. The combination of gears and speed make oil a necessity. Oil lubricates the interior of the lower unit housing to prevent it from overheating due to friction and seizing. Without oil, your day on the water would be cut short. 

How Often Should You Change Lower Unit Oil

If you’re wondering how often to change lower unit oil, remember to consult your service manual, but the general rule of thumb is changing gear oil in the lower unit every 100 hours. Take note of the hour meter after each change and monitor the meter until 100 hours have been reached. Oil must remain fresh because of the heavy load enforced upon them. 

How Much Oil Is In A Lower Unit

The amount of oil is highly impacted by the horsepower rating of the outboard motor. The greater the horsepower, the greater the gear oil capacity and vice versa. Generally speaking, outboards require between six and forty-five ounces of marine lower unit oil. Consult your engine manual for exact measurements. 

How Do You Change The Oil In A Boat Lower Unit

The process of changing the oil in an outboard lower unit is straightforward. Gather your supplies, including fresh oil, flat head screwdriver, drain pan, rags, and gear oil pump.

Lower The Engine

Trim the engine in the down position so old oil does not fail to drain from the lower unit of the boat motor. 

Remove The Top Screw

Remove the screw at the upper portion of the lower unit. The upper screw allows air to enter the gear housing thus increasing the flow rate when the lower screw is backed out. 

Remove The Bottom Screw

Position the drain pan beneath the lower unit and unscrew the lower screw. Stow the upper and lower screws in a convenient location that is easily accessible. 

Position The Pan And Drain

With the lower and upper screws removed, the oil will drain from the gear case completely. The process takes between five and ten minutes. 

Connect The Gear Oil Pump

Screw the gear oil filling tube to the lower drain plug hole. Connect the pump to the bottle of oil. Pump the oil into the lower unit until the air escapes from the upper screw hole and pours over the side. 

We recommend the Performance Tool W54150 Marine Lower Unit Fluid Pump. The pump comes with two adapters and a clear 24-inch hose. 

Install The Top Screw

With the oil pump hose secured to the drain/fill port, install the upper screw. The installation of the screw prevents oil from pouring rapidly from the bottom when the hose is removed. 

Install The Lower Screw

Once the fill hose is unscrewed and removed, replace the hose with the lower screw as quickly as possible without cross threading. 

What Color Should My Lower Unit Oil Be Before Adding To The Motor

The color ranges are based on the brand of outboard motor gear oil purchased. However, the most common color is blue, but yellow, green, and brown are not uncommon. Whatever the color, you know the oil is fresh when pumped from a sealed bottle. 

What Color Should My Lower Old Lower Unit Oil Be After Removing From The Motor

The drained lower unit gear oil should be black or dark brown. If the outboard motor leg oil is milky white or gray, this is an indication of a serious lower unit issue. 

Milky coloration indicates water in lower unit oil, while gray suggests a mechanical issue due to grinding metal, thus creating shavings. When either a present, immediately have the lower unit inspected. 

Now You Know The Importance Of Lower Unit Oil

As the saying goes, don’t become dead in the water. With proper maintenance, the lower unit will last for extended periods. Failure to change boat gear oil routinely results in costly and avoidable repairs. The simple process can be performed in a driveway, marina maintenance rack, or garage. Boating is expensive, but gear oil changes are not. Keep up with your lower unit to remain on the water.