Marine Fuel Additive Buying Guide: Stabilizers, Ethanol Treatment and Diesel

By Boat Supplies Direct·Updated Oct 18, 2026·5 min read

Fuel water separator filter mounted in the stern of a boat with twin outboards

Quick answer: For gasoline boats running ethanol blended (E10) fuel, use a marine fuel treatment or stabilizer at every fill-up, not just before winter. For storage, treat the fuel, fill the tank to about 90 to 95 percent, and run the engine long enough to pull treated fuel through the system. Diesel boats need a diesel-specific treatment. And no additive can reverse phase separation once it has happened.

Boat fuel lives a harder life than car fuel. It sits for weeks, breathes humid air through the tank vent, sloshes around and gets hot in the sun. Most gasoline is E10, up to 10 percent ethanol, and ethanol attracts water. The result is gum, varnish, corrosion, clogged injectors and the most frustrating problem of all, phase separation. Fuel additives prevent most of these problems when used consistently. This guide explains what each type does and how to use them.

Step 1: Understand the problem with ethanol

Ethanol blends absorb moisture from the air. E10 can hold a small amount of water, roughly half a percent, mixed in. Past that point the ethanol and water drop out of the gasoline and settle to the bottom of the tank as a separate layer. That is phase separation. The bottom layer is corrosive and will not burn properly, and the gasoline above it has lost octane. If the pickup draws from the bottom layer, the engine stumbles, stalls or worse.

Also, never use E15 or higher in a marine engine. Marine engines are generally not approved for it, and it can void warranties.

Step 2: Match the additive to the job

Additive type	What it does	When to use
Fuel stabilizer	Slows oxidation, prevents gum and varnish	Any fuel that sits more than a few weeks, and every winterization
Ethanol treatment	Helps manage moisture, fights corrosion from ethanol blends	Every fill with E10
Enzyme treatment	Breaks down sludge and helps keep water dispersed	Every fill, especially boats used irregularly
Injector / fuel system cleaner	Removes deposits from injectors, carbs and valves	Periodically, or if the engine runs rough
Diesel treatment	Stabilizes diesel, improves cetane, fights sludge	Every fill and for storage in diesel boats

Many marine products combine several of these. Read the label to know what you are getting.

Step 3: Dose it right

Most treatments are dosed per gallon. Add the treatment to the tank before fueling so the incoming fuel mixes it thoroughly. Overdosing does not make it work better. Keep a bottle on board so you treat at every fill, not just when you remember.

Step 4: Winterize and store properly

Treat the fuel with stabilizer at the storage dose on the label.
Fill the tank to about 90 to 95 percent. A nearly full tank leaves less air space for condensation, while a small gap allows for expansion.
Run the engine for 10 to 15 minutes so treated fuel reaches the fuel lines, filters, injectors or carburetor.
Replace or drain the fuel/water separator filter.

Some owners prefer storing with a nearly empty tank. With ethanol fuel, a full, treated tank is the common recommendation from engine makers, but check your engine manual.

Step 5: Your fuel/water separator is the backup

Additives reduce problems, but a fuel/water separating filter catches water and debris before they reach the engine. Most outboard makers recommend a 10-micron separator between the tank and the engine. Check the bowl regularly and change the element at least once a season.

Step 6: If phase separation has already happened

Additives cannot fix it. The water and ethanol layer has to be pumped out of the tank, the filters replaced, and the tank refilled with fresh fuel. This is why prevention matters.

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Frequently Asked Questions

Do I need fuel stabilizer for my boat?

Yes if your boat sits for more than a few weeks at a time or over the off-season. Stabilizer prevents fuel from oxidizing into gum and varnish that clog the fuel system.

What is phase separation in boat fuel?

When ethanol-blended gasoline absorbs more water than it can hold, the ethanol and water separate and sink to the bottom of the tank. That layer is corrosive and does not burn properly.

Can a fuel additive fix phase separation?

No. Once fuel has phase separated, the separated layer must be removed from the tank. Additives help prevent it, they do not reverse it.

Should I add fuel treatment every time I fill up?

For boats running E10, yes. Treating at every fill keeps fuel stable and protects the system, instead of reacting after problems start.

Should I store my boat with a full or empty tank?

Most engine makers recommend a treated tank filled to about 90 to 95 percent to limit condensation, leaving room for expansion. Follow your engine manual.