Before you get started, remember that all manufacturers have their own oil and filter recommendations and ratings, so check your owner's or service manual to make sure you have the right materials.
With inboard engines, treat the fuel with a stabilizer and an additive aimed at combating ethanol-related problems, then run the engine for a long enough period of time so the mixture gets through to the engine. Next, change all the fuel filters, adding a treatment/stabilizer into the filters. This helps get it into the system quicker.
If the engine has an electric fuel pump it's important to fill all fuel filters with gas. You can damage the fuel pump by running it dry.
If the inboard has a closed cooling system, check the antifreeze for the right degree of protection. Check all fluids, such as power steering and transmission, and fill to appropriate levels. Clean all seawater strainers.
Next, make sure you have a water supply going to the raw water pump and start the engine. You can damage the impeller in as little as 30 seconds by running the pump dry. By running the engine you are flushing the raw water system and warming the oil so it can be extracted easily. Most engines have a tube or hose to draw the oil out with a pump, and this works better when the oil is warm.
When the engine gets to normal operating temperature, change the oil and filter, filling the filter with oil before installing it, if possible. Start the engine and check for leaks; check the oil level. (Click here for tips on changing the oil.)
The next step - protecting the engine internally for storage - can be a little tricky. Carbureted and electronically fuel-injected engines have different fogging methods. With a carbureted engine, you spray the fogging oil into the engine through the carburetor. With an EFI engine, you put the winterization mixture in the fuel filter or a separate tank, but check your manual for the recommended procedure.
The last step is to drain the raw water system and fill it with non-toxic antifreeze that has rust inhibitors to protect the metals. When you're finished, lube the grease fittings, linkages, cable ends and anything that moves (no belts or pulleys) so they won't seize up in storage. Use a good water-dispersing spray on the outside of the engine, and you're done. Don't forget to block the exhaust so that critters don't make a home in it.
With a diesel, the process is basically the same, though fogging isn't necessary. Flush the engine, stabilize the fuel, change the oil and filters, and top off all fluids. Run the engine to get the treated fuel through, and finish by using a water dispersing spray on the exterior.
NOTE: The generator is winterized the same way as the main engine but on a smaller scale.
Sterndrive boats follow the same procedures for inboards, with additional care for the outdrive:
Start by stabilizing and treating the gas and changing and/or cleaning all the filters. Remember to pump the primer bulb on EFI outboards so that the electric pump doesn't run dry.
Next, change the lower unit gear oil. Always change the gasket (or O-ring) on the drain plug. Remove the propeller and grease the splines. You can leave the prop off or put it back on.
Start the engine to flush it. Remember to have water going to the pump.
There are four types of outboard engines - carbureted or fuel-injected 2-stroke and carbureted or fuel-injected 4-stroke - and there are different methods of protecting them internally. With a carbureted 2-stroke, let the engine warm up and spray fogging oil through the carburetor until the engine stalls. Remove the spark plugs and spray fogging oil into each cylinder, then reinstall the spark plugs. For EFI 2-strokes and 4-strokes and carbureted 4-strokes, you must consult the manual because some can be fogged while others require an oil mixture be put in the fuel filter or a separate tank.
No matter how the manufacturer specifies it be done, the goal is the same: to protect the internal components of the engine. The main difference with a 4-stroke is that the oil and filter must be changed.
Finally, grease and lube all fittings and linkages, and wrap a bag around the prop to stop critters from getting into the exhaust. Use a water dispersing spray on the exterior. Do not spray any belts or the starter Bendix.