BOAT WINTERIZATION:
READY FOR THE OFF-SEASON.

Start with your boat’s exact setup.

Record the engine make, model, serial number, fuel type, hours, and drive model. Find the storage sections in both the engine and boat manuals. Use this guide to organize the work; the manuals determine quantities, drain locations, torque values, and the order of service.

• Four-stroke gasoline outboard: plan engine-oil/filter and lower-unit service, plus the specified fuel, internal-protection, and draining procedures.
• Two-stroke outboard: follow its injection or premix requirements. Direct-injected engines can have different internal-protection instructions; some engines have an automatic storage routine.
• Inboard or sterndrive: identify every raw-water cooling circuit, including engine, exhaust, and accessories. A closed cooling system can still have raw-water components.
• Diesel: use the diesel engine’s layup procedure and oil specification. The gasoline stabilizer and gasoline fogging discussion below are not a diesel treatment plan.

Gather the specified fluids, filter, seals, drain pan, absorbent pads, tools, and approved flushing equipment. Arrange marine service before freezing weather if you cannot identify the cooling circuits or complete the manufacturer’s procedure.

A practical boat winterization plan.

1. Prepare the fuel for storage.

Follow your tank and engine manufacturer’s instructions for fuel level, treatment, and any carburetor draining. Tank designs differ, so “fill every tank to the brim” is not a universal rule. Leave the required expansion space. Circulate treated fuel as directed using an approved cooling-water supply.

AMSOIL Gasoline Stabilizer (AST) is intended for stored gasoline in two- and four-stroke engines, including ethanol blends. It helps limit deterioration and gum formation. Measure from the current label for the amount of fuel being treated. Stabilizer is preventive maintenance; have stale or contaminated fuel addressed before storage.

2. Service the engine oil and filter.

For a four-stroke, complete the oil/filter service called for before storage. Match the oil’s viscosity and marine specification to the engine, use the correct filter and seals, and verify the final level and leaks as instructed. Do not assume a two-stroke needs the same crankcase oil change.

AMSOIL’s 10W-30 (WCT) and 10W-40 (WCF) Synthetic Marine Engine Oils are gasoline four-stroke options where the specified grade and application match. These products carry NMMA FC-W Catalyst Compatible certification. Choose the grade your engine calls for, rather than changing viscosity just because winter is coming.

3. Inspect and service the lower unit or drive.

Inspect drained lubricant for water or abnormal metal. Milky oil or free water calls for diagnosis of the drive and seals; fresh oil alone does not fix a leak. Refill and seal the drive using its specified procedure and lubricant. Collect used oil for proper recycling.

AMSOIL 75W-90 Synthetic Marine Gear Lube (AGM) is a candidate for applications calling for 75W-90 or 80W-90 gear lube and the specifications AMSOIL lists. Confirm the exact drive: some marine transmissions require different fluids.

4. Protect engine internals using the correct method.

AMSOIL Engine Fogging Oil (FOGSC) coats internal components to help limit storage corrosion. Use it only where the engine’s instructions permit it, with the specified application method. Do not improvise by spraying into an EFI or direct-injection intake. Check for an automatic winterization routine or a required alternative oil procedure.

Engine-service reference: Mercury Marine’s outboard winterization guide. Product applications and directions: the AMSOIL links above.

5. Complete freeze protection throughout the boat.

Flush and drain cooling passages as directed. For outboards, use the manufacturer’s specified storage position so trapped water can escape. For inboards and sterndrives, follow the full draining/antifreeze procedure for the particular engine and exhaust system. Seeing colored liquid at the exhaust does not establish that every passage is protected.

Include livewells, washdown pumps, bilge equipment, freshwater plumbing, heads, and any installed generator or air-conditioning raw-water circuit. Winterizing antifreeze must be approved for the particular system and expected temperatures. Engine coolant and potable-water winterizing fluid are different products; never put engine coolant in drinking-water plumbing.

Freeze-protection reference: BoatUS Boater’s Guide to Winterizing (PDF).

6. Lubricate, inspect, and protect.

Use the manual’s lubrication-point list. Inspect the propeller area for fishing line and damage, and check anodes and corrosion. For suitable grease points and serviceable trailer bearings, AMSOIL Synthetic Water-Resistant Grease (GWR) is formulated to resist washout in wet service. Confirm grease compatibility and the bearing service procedure before adding or changing grease.

7. Make a battery and storage plan.

Follow the battery maker’s storage, charging, and temperature limits for its chemistry. Clean and dry the boat, support a ventilated cover, and inspect it periodically for pooled water, snow load, and pests. For a boat stored afloat, keep required bilge-pump power and monitoring in service; disconnecting all batteries is not a universal storage instruction. Keep a written list of removed plugs, disconnected hoses, and work still needed.

Storage references: AMSOIL’s boat and motor guide and Mercury Marine’s boat-storage guide.

Choose the AMSOIL product for the job.

These are options to compare against your manuals, not a universal winterization kit. Open each product page for current specifications, label directions, package sizes, price, and availability. Every shopping link below includes Clearwater Oil dealer number 30575602.

Browse Clearwater Oil’s marine engine-oil catalog · Check AMSOIL application guides ↗

Leave a clear plan for spring.

Use your storage record to work through the manufacturer’s return-to-service procedure. Account for every removed plug, hose, and cover; restore the required connections and cooling-water supply before starting. Verify fluid levels, battery condition, and leaks. Check the boat’s safety equipment and trailer before the first trip.

Record the service date, engine hours, products and quantities used, unresolved repairs, and next service due. Give that record to the shop if someone else handles commissioning.