1. Run the engine up to operating temperature and stop.
2. Drain the engine oil through the sump plug into a container, or pump oil out through the dipstick tube using the hand pump if fitted. If you use a separate pump, push the tube down until you feel it touch the bottom of the sump.
3. Remove the old oil filter, fill the new one with engine oil and fit as tight as you can by hand, first smearing some oil on the rubber sealing ring.
4. Fill the engine with clean oil to the correct dipstick level.
5. Run engine, stop, check the oil level again, and retighten the filter.
IMPORTANT. Waste or spilled oil should always be disposed of in designated receptacles ashore, never in the sea, river or canal.
Some engines share their lubricating oil with the gearbox, in others the two systems are separate. In the clutch-type gearbox the normal lubrication recommended is Automatic Transmission Fluid, or ATF.
In all cases refer either to the engine manufacturer or the gearbox manufacturer for the fluid recommended.
Heat is generated in the gearbox through the end loading imposed by the propeller shaft on the bearings, and where high powers are being transmitted the oil needs to be cooled. This is usually achieved with a heat exchanger cooled by water from the raw water engine cooling system.
Always use clean gearbox oil, as there are no filters incorporated in the system. Fill to the recommended level shown on the dipstick or the fill plug. Be careful, also, not to lose the plastic fill pipe often supplied with the plastic oil container; it will invariably disappear into the gearbox. A funnel is the answer.
Changing the oil (clockwise from top left). 1. Using the built-in pump to change the oil, typically about a gallon for a 30hp engine. 2. A strap wrench makes easy work of removing the oil filter. Alternatively, and messier, spear the filter with a screw-driver and twist. 3. A plastic bag will catch most of the leaks. A smear of oil will help bed the new filter. 4 and 5. After filling and running the engine, recheck oil level and top up if necessary. Dispose of oil and filter responsibly.
The gearbox will have a separate dipstick and may well share the same grade of oil as the engine. Gearbox oil is not filtered during use. It not only lubricates the gears but cools the box, often along with a water jacket.