Short answer
On many outboards you drain through the lower plug and fill from below until oil comes out of the level hole. Sterndrives are often filled and level-checked in other ways, so follow the manual for your model. Use the oil it specifies and new seals for the plugs. The Mercury 15/20 EFI FourStroke needs new oil every 100 hours or once a year, whichever comes first. Change it before winter storage so no water is left in the gearcase.
Quantity and oil type from the owner's manuals
| Model | Oil | Quantity |
|---|---|---|
| Volvo Penta SX, per 7737514 | Synthetic GL-5 SAE 75W-90 | 2.1 l |
| Volvo Penta SX-A, per 47702420 | Synthetic GL-5 SAE 75W-90 | 2.65 l |
| Volvo Penta DP-S, per 7737514 | Synthetic GL-5 SAE 75W-90 | 2.4 l |
| Tohatsu MFS4B/5B/6B | Tohatsu gear oil or API GL-5 SAE 80 to 90 | Approx. 195 ml |
With a spacer on the drive or a different variant of the model, the quantity will differ. See the drive catalogue, MerCruiser Alpha One Gen II and the outboard manuals in the engine catalogue. The litre figures help you buy the right amount. When filling, the level hole or dipstick is what counts.
How to do it on an outboard with a lower fill hole
Make sure the engine can't start before you work near the propeller.
- Set the engine upright and put a container underneath.
- Find the drain and level plugs in the manual. Unscrew them and let the oil drain out.
- Look at the oil and at the magnet, if the plug has one.
- Pump new oil in through the lower hole until it comes out of the upper level hole.
- Screw in the level plug with a new seal while the pump is still attached. Then remove the pump and fit the lower plug with a new seal.
- Tighten the plugs to the torque given in the manual and check that nothing leaks.
A drive with a different filler opening, dipstick or separate oil reservoir is filled its own way.
What the oil tells you
- Milky or grey: water has probably got in. Have a workshop check the seals and pressure test the gearcase.
- Metal on the magnet: look at how much there is and how big the pieces are. Larger flakes, or a new noise, mean the gearcase needs inspecting.
- Less oil than expected: look for the leak and find out what the level was last time. Just topping up solves nothing.
Have at home
- The right oil and a pump that fits the hole.
- New plug seals and a torque wrench.
- A container and rags or absorbent for spills.
You'll find part numbers in the spare parts catalogue. Note the date, engine hours, oil type and what you found in the service log. Take a photo of the old oil. If it looks odd, the workshop will find the photo useful.
Frequently asked questions
How often should I change the gearcase oil?
As the owner's manual says, usually once a season or after a set number of hours. Change it before winter storage so that any water in the gearcase can't freeze.
Why do you often fill from the bottom?
When the oil comes in from below, it pushes the air up and out through the level hole. Still, do what the manual says for your model.
Can I use engine oil in the sterndrive?
Only if the manual for your particular drive says so. Don't go by brand or by how the oil looks. Different drives can require different oils.
The oil is milky. Can I run the engine?
Change the oil and have the seals checked before you run it any more. Water in the oil wears out bearings and gears.
Sources
- Volvo Penta Owner’s Manual 3.0–5.8 GL/GS/Gi/FL/Fi (7737514), tekniska data drev pubs.volvopenta.com
- Volvo Penta Operator’s Manual 5.0L/5.7L, SX-A/DPS-B (47702420) pubs.volvopenta.com
- Mercury 15/20 EFI FourStroke, underhåll och växelhusolja servicelit.mercurymarine.com
- Tohatsu MFS4B/5B/6B, specifikationer och växelhusolja tohatsu.com