Short answer
On many outboards you drain through the lower plug and fill from the bottom until oil reaches the level hole. Sterndrives may have different fill and level checks: always follow the manual for your exact model. Use the specified oil and new plug seals. Mercury 15/20 EFI FourStroke specifies a change every 100 hours or annually, whichever comes first. Check the oil before winter storage so that any water isn't left in the gearcase.
Quantity and oil type – examples 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 | Specified gear oil or API GL-5 SAE 80–90 | Approx. 195 ml |
Sources: the listed Volvo Penta manuals and Tohatsu's specification table. Drive spacers and model variant may change the quantity. See the drive catalog, MerCruiser Alpha One Gen II and the outboard manuals in the engine catalog. The volume is a planning figure; use the manual's level check when filling.
Example: outboard with lower fill hole
Follow the model's safety instructions and prevent accidental starting before working near the propeller.
- Set the engine vertical and place a drip tray underneath.
- Identify the correct drain and level plugs in the manual. Remove them and let the oil drain out.
- Inspect the oil and any magnet on the plug.
- Pump the specified oil in through the lower hole until it reaches the upper level hole as per the manual.
- Fit the level plug with a new seal before removing the pump, then the lower plug with a new seal.
- Tighten to the model's specified torque and check for leaks.
Don't automatically apply this method to a drive with a different filler opening, dipstick or separate oil reservoir.
What the oil tells you
- Milky or grey: water may have got in. Have the seals inspected and the gear housing pressure-tested.
- Metal on the magnet: assess the amount and particle size. Larger chips or a new noise call for an inspection.
- Unexpectedly low amount: check the previous level and look for possible leaks; don't just top up and forget the cause.
Have at home
- The specified oil and a pump with the right fitting.
- New plug seals and a torque wrench.
- A drip tray and material to deal with spills.
Check part numbers in the parts catalog. Record the date, engine hours, oil specification and any findings in the service log. A photo of the drained oil can help the workshop if something looks unusual.
Frequently asked questions
How often should I change the gearcase oil?
As specified in the owner's manual, usually once a season or after a set number of hours. Change it before winter storage so that water in the gear housing can't freeze.
Why do you often fill from the bottom?
On designs with a lower filler hole, this helps the oil push air up and out through the level hole. Still, follow the filling method for your model.
Can I use engine oil in the sterndrive?
Only if the instructions for your specific drive say so. Don't guess based on make or appearance; different drives may require different oils.
The oil is milky – can I keep running?
Change the oil and check the seals before running it any further. 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