The 7 Best Fuel Additives for Old Gas – Save Your Engine in 2025

If you've discovered old gas in your vehicle or equipment, you're facing a common but serious problem that can damage your engine. While it's tempting to simply top off the tank and hope for the best, using the right fuel additive can mean the difference between a smoothly running engine and costly repairs. From preventing phase separation to removing harmful deposits, today's fuel stabilizers offer specialized solutions for aging gasoline. Let's explore the top seven additives that'll protect your investment and keep your engine running at its peak in 2025.

STA-BIL Storage Fuel Stabilizer, 4 fl. oz. (22205)

If you're looking to store fuel for up to 24 months, STA-BIL Storage Fuel Stabilizer stands out as a top choice for protecting your engines. You won't need to drain your fuel tanks before storage, and you'll get quick starts when you're ready to use your equipment again.

This fuel stabilizer prevents gum and varnish build-up while protecting your entire fuel system from corrosion. It's effective in all gasoline types, including ethanol blends, and treats up to 80 gallons per 32 fl. oz. bottle. You'll find it particularly useful for lawnmowers, generators, boats, and recreational vehicles.

Users consistently report smooth engine starts after winter storage and praise its effectiveness in maintaining fuel integrity. At an affordable price point, STA-BIL provides exceptional value by preventing costly repairs related to fuel system degradation.

Best For: Ideal for homeowners and recreational vehicle owners who need to store gas-powered equipment or vehicles for extended periods, particularly during seasonal transitions.

Pros:

• Keeps fuel fresh for up to 24 months, eliminating the need to drain tanks before storage
• Works effectively with all gasoline types including ethanol blends
• Prevents expensive repairs by protecting against corrosion and fuel system degradation

Cons:

• 4 fl. oz. size may require multiple bottles for treating larger fuel volumes
• Regular reapplication needed for optimal protection
• Slightly higher cost compared to generic fuel stabilizers

STAR BRITE Star Tron Enzyme Fuel Treatment (93008SS)

Car owners seeking a versatile fuel treatment will appreciate STAR BRITE Star Tron's concentrated enzyme formula, which treats up to 128 gallons with just 8 ounces of product. This powerful additive stabilizes fuel for up to two years while preventing phase separation by dispersing water into tiny droplets.

You'll find it's incredibly easy to use – just add 1 oz. per 8 gallons initially, then 1 oz. per 16 gallons for maintenance. It's compatible with all engines and fuels, making it perfect for your car, boat, motorcycle, or lawn equipment. The enzyme formula effectively tackles ethanol-related problems, removing gum, carbon deposits, and varnish build-up.

With over 3,000 reviews and a 4.7-star rating, users consistently report improved fuel economy, reduced emissions, and restored performance in old engines. It's particularly effective for maintaining emergency generators and small engines sensitive to ethanol.

Best For: Vehicle and equipment owners looking for a comprehensive fuel treatment that prevents ethanol issues, stabilizes fuel long-term, and improves engine performance across multiple types of engines.

Pros:

• Highly concentrated formula treats up to 128 gallons and provides up to 2 years of fuel stabilization
• Compatible with all engine types and can be mixed with other fuel additives without risk
• Strong track record with excellent user reviews and proven effectiveness in preventing ethanol-related problems

Cons:

• Initial treatment requires higher dosage which means using more product
• Premium price point compared to basic fuel stabilizers
• May need regular reapplication for optimal results in frequently used vehicles

Gumout Multi-System Tune-Up Fuel System Cleaner (16 oz)

Reviving stale fuel becomes effortless with Gumout Multi-System Tune-Up Fuel System Cleaner, a versatile 16 oz solution that's proven effective for both automotive and small engine applications. This extensive cleaner tackles carbon buildup, corrosion, and fuel system deposits while acting as a stabilizer for your aged fuel.

You'll find it works effectively in gas, diesel, and ethanol engines, with clear dosage instructions for different applications. For cars, add the entire bottle to treat 18-21 gallons of fuel, while small engines need just ¾ to 1 ounce per gallon. The PEA-based formula delivers noticeable improvements in engine performance and helps reduce emissions. For best results, you'll want to use it every 7,000-10,000 miles in vehicles or with each tank in lawn equipment. It's particularly useful for seasonal storage to prevent fuel degradation.

Best For: Ideal for vehicle owners and small engine operators looking to revive stale fuel, maintain engine performance, and protect their fuel systems during storage periods.

Pros:

• Versatile application across multiple engine types (gas, diesel, ethanol) and sizes (cars to lawnmowers)
• Contains premium PEA cleaning agents that effectively remove carbon deposits and combat corrosion
• Clear dosage instructions for different applications and serves multiple functions (cleaner, stabilizer, performance enhancer)

Cons:

• Can design may make precise pouring difficult for small engine applications
• Higher price point compared to basic fuel additives
• Requires regular reapplication for optimal results (every 7-10k miles or each tank for small engines)

Rislone Hy-per Fuel Complete Fuel System Cleaner Gas

Motorists seeking an all-encompassing solution for old gas will find exceptional value in Rislone Hy-per Fuel Complete Fuel System Cleaner. This EPA-registered product combines six essential functions: injector cleaning, fuel system cleaning, upper cylinder lubrication, fuel stabilization, moisture removal, and octane boosting.

You'll appreciate how this multifunctional additive enhances your vehicle's performance while tackling old gas issues. It's proven to improve fuel economy and reduce common problems like rough idle, hard starting, and hesitation. With a impressive 4.6-star rating from over 1,300 reviews, it's clear why this product ranks #15 in fuel additives. Add it every 5-10 fuel tank fills for ideal results, and you'll notice improvements within just a few miles of driving. It's safe for oxygen sensors and catalytic converters, making it suitable for all modern vehicles.

Best For: Drivers looking for a comprehensive fuel system treatment that can address multiple issues including old gas, poor performance, and fuel system cleaning while being safe for modern vehicles.

Pros:

• Six-in-one formula provides multiple benefits in a single product, including cleaning, lubrication, and octane boosting
• EPA-registered and safe for use with oxygen sensors and catalytic converters
• Strong customer satisfaction with 4.6/5 stars from over 1,300 reviews

Cons:

• Requires regular application every 5-10 tank fills for optimal results
• May take several miles of driving before noticing improvements
• Price point might be higher than single-function fuel additives

Star brite 14316 Star Tron Enzyme Fuel Treatment – 16 oz.

For owners of equipment that sits idle for long periods, Star Tron Enzyme Fuel Treatment offers a powerful solution to prevent fuel degradation and engine problems. This versatile fuel additive works with all engine types and fuel varieties, making it ideal for boats, cars, motorcycles, lawn equipment, and generators.

You'll appreciate how Star Tron's enzyme technology tackles multiple issues at once. It stabilizes fuel for up to two years, prevents phase separation by dispersing water into tiny droplets, and eliminates carbon deposits and varnish buildup. The treatment's easy dosage requires just 1 oz per 3 gallons initially, then 1 oz per 6 gallons for maintenance. What sets it apart is its ability to revitalize old fuel while maximizing fuel economy and reducing harmful emissions. Whether you're dealing with ethanol-blended or ethanol-free fuel, Star Tron keeps your engines running smoothly.

Best For: Equipment owners who need long-term fuel stabilization and protection against ethanol-related issues, particularly those with vehicles or machinery that sits idle for extended periods.

Pros:

• Stabilizes fuel for up to 2 years while preventing phase separation and water-related problems
• Compatible with all engine types and fuel varieties, including ethanol-blended fuels
• Improves fuel economy and reduces emissions by preventing carbon deposits and varnish buildup

Cons:

• Initial treatment requires double the maintenance dose, which may be confusing for some users
• Higher price point compared to basic fuel stabilizers
• May need consistent, long-term use to see optimal results in older engines

Hot Shots Secret Gasoline Extreme Fuel System Cleaner (32 oz)

Hot Shot's Secret Gasoline Extreme delivers powerful cleaning performance for drivers battling aging fuel systems. This concentrated 32 oz formula uses advanced P.E.A. technology to dissolve gum, varnish, and deposits throughout your engine's fuel system, from injectors to combustion chambers.

You'll notice up to 7.9% improvement in fuel economy and enhanced engine power after adding 12 oz per 20 gallons of gas. It's especially effective at smoothing rough idles and restoring performance in troubled engines. The formula cleans and protects essential components like valves, pumps, rings, and pistons while improving combustion efficiency. With a strong 4.7-star rating from over 700 customers, it's proven its worth across various vehicle types. Use it during refueling or every 10,000 miles to maintain peak engine performance and protect against the effects of old gas.

Best For: Car owners seeking to restore performance and fuel efficiency in aging or underperforming gasoline engines, especially those experiencing rough idling or reduced power.

Pros:

• Significant fuel economy improvement of up to 7.9% with regular use
• Comprehensive cleaning action that addresses multiple engine components from injectors to combustion chambers
• Strong customer satisfaction evidenced by 4.7/5 stars from over 700 reviews

Cons:

• Requires relatively large amount (12 oz per 20 gallons) compared to some competitors
• Premium price point compared to basic fuel system cleaners
• May need multiple treatments for severely contaminated fuel systems

Lucas Oil 10977 High Mileage Fuel Treatment – 5.25 Ounce

Power-hungry engines in high-mileage vehicles find their match in Lucas Oil 10977 High Mileage Fuel Treatment. This 5.25-ounce bottle packs a potent formula that'll clean your fuel injectors while reducing deposits in the intake valves and combustion chamber.

You'll appreciate how it tackles those frustrating engine issues – from knocking to hesitation – while providing essential lubrication to critical fuel system components. It's particularly effective in modern port-injected engines and won't harm your oxygen sensors since it's alcohol-free.

With a stellar 4.7-star rating from 678 users, this USA-made treatment has proven its worth in restoring lost power and performance. You can add it after every oil change or during regular fill-ups. While some users haven't seen dramatic mileage improvements, they consistently praise its ability to maintain engine cleanliness and smooth operation.

Best For: High-mileage vehicle owners looking to maintain engine performance, reduce deposits, and address issues like knocking and hesitation in their aging engines.

Pros:

• Effectively cleans fuel injectors and reduces engine deposits without containing harmful alcohols
• Strong customer satisfaction with 4.7/5 stars from 678 ratings
• Versatile application for both regular maintenance and addressing specific engine performance issues

Cons:

• Some users report minimal improvement in fuel economy
• Relatively small bottle size at 5.25 ounces may require frequent purchases
• Higher price point compared to basic fuel additives

Factors to Consider When Choosing Additives for Old Gas

When selecting additives for old gas, you'll need to evaluate critical factors like the fuel's age, current condition, and whether your engine type is compatible with the treatment. You should consider how long you'll store the treated fuel and understand the proper mixing ratios to avoid over or under-treating your gas. It's also essential to check if your additive can handle the fuel's ethanol content, as most modern gasoline contains ethanol that can attract moisture and cause stability issues.

Fuel Age and Condition

Understanding your fuel's age and condition is critical before selecting any additive treatment. If your gas has been sitting for more than three months, you'll likely notice significant degradation that requires immediate attention. The presence of ethanol in most modern fuels accelerates this deterioration process, as it attracts moisture and can lead to phase separation.

You'll want to assess several key indicators of fuel condition. Check for changes in color, clarity, and smell. If your fuel appears darker than usual or has visible sediment, it's showing signs of degradation. You might also notice poor engine performance, including hard starts or rough idling, which often indicates that your fuel has lost its volatility.

Pay special attention if you're dealing with ethanol-blended gasoline, as it's particularly susceptible to water contamination. This can cause phase separation, where water and fuel separate into distinct layers, potentially damaging your engine's components. Before adding any treatment, you'll need to determine whether your fuel shows signs of water contamination, as this will influence which additive type you should choose to restore fuel quality and protect your engine.

Engine Type Compatibility

Your engine type serves as the primary factor in selecting the right fuel additive for old gas. Whether you're dealing with a gasoline or diesel engine, you'll need to choose an additive specifically formulated for your engine's requirements to guarantee peak performance and prevent potential damage.

If you're treating old gas in small engines like lawn mowers or generators, look for additives that specifically address ethanol-related problems and enhance fuel stability. These products will help protect your equipment's fuel system from the harmful effects of degraded fuel. For high-mileage engines, you'll want to focus on additives that combat carbon deposits while providing essential lubrication to aging components.

Don't forget to check if the additive is compatible with your engine's oxygen sensors and catalytic converter. Using the wrong product could damage these expensive parts or void your warranty. If you're working with equipment that uses 2-cycle or 4-cycle engines, make sure the additive you choose is specifically designed for your engine type. Mixing up these formulations could lead to poor performance or even engine damage.

Storage Duration Requirements

Storage duration stands as a critical factor in selecting the right fuel additive for old gas. When you're choosing an additive, you'll want to take into account how long you need to store your fuel, as different products offer varying levels of protection. Many high-quality additives can stabilize fuel for up to 24 months, which is ideal if you're storing vehicles or equipment for extended periods.

You'll need to pay special attention to phase separation issues, particularly if you're using ethanol-blended fuels. Look for additives that specifically address this concern, as water contamination can occur during lengthy storage periods. The best products will prevent the need to drain your fuel tank before storage, saving you time and money while ensuring your engine starts reliably after periods of inactivity.

If you're dealing with already deteriorated fuel, select additives designed to revitalize sub-spec gas. Take into account the application frequency as well – some additives require regular treatment every few months, while others work effectively with each refueling. This maintenance schedule should align with your storage needs and usage patterns to provide ideal fuel protection.

Treatment Concentration Ratios

Precise measurement remains essential when adding fuel treatments to old gas, as proper concentration ratios directly impact their effectiveness. Most fuel additives require 1 ounce of treatment for every 8 to 16 gallons of gasoline, though you'll need to check your specific product's instructions for exact measurements.

When you're dealing with severely degraded fuel, you might need to use a higher initial concentration to effectively treat the old gas. After this initial treatment, you can switch to a lower maintenance dose for ongoing protection. You'll want to calculate the exact amount of additive needed based on your fuel tank's capacity or the volume of gas you're treating to avoid over or under-treating.

Don't be tempted to add extra additive thinking it'll work better – overdosing can damage your fuel system or cause performance issues. Following the manufacturer's recommended ratios isn't just a suggestion; it's vital for your engine's health. By maintaining proper concentration levels, you'll maximize the additive's benefits while ensuring your old gas becomes stable and usable again.

Ethanol Content Management

Beyond proper measurement, understanding ethanol content stands as a key factor when choosing additives for old gas. If you're dealing with ethanol-blended fuels, you'll need additives specifically designed to combat water absorption and phase separation. These specialized additives help disperse water throughout the fuel mixture, preventing it from settling at the bottom of your tank where it can cause significant damage.

When selecting a fuel additive, you'll want to take into account your engine's age and design specifications. Older engines weren't built to handle high ethanol concentrations, so you'll need products that can effectively manage ethanol's corrosive properties and prevent gum and varnish buildup in your fuel injectors. Look for additives that offer both cleaning and lubricating properties to protect your fuel system.

For long-term storage, choose a fuel stabilizer that's specifically formulated for ethanol-blended fuels. These products can extend your fuel's shelf life up to two years while maintaining its integrity. Remember to check the ethanol percentage in your fuel regularly, as higher concentrations will require more frequent additive treatments to maintain ideal engine performance.

Cost Per Treatment Value

When shopping for fuel additives, you'll need to look beyond the sticker price to determine true value. A $10 bottle that treats 128 gallons of fuel offers considerably better value than one treating only 20 gallons at the same price. To calculate the actual cost per treatment, divide the product's total price by the number of treatments it provides.

Consider the recommended usage frequency when evaluating long-term costs. Some additives require application every 5,000 miles, while others need adding at each fill-up. This difference can dramatically impact your annual spending on fuel treatments. Look for multi-purpose additives that combine fuel stabilization with cleaning properties, as they'll give you more bang for your buck by eliminating the need to purchase separate products.

Don't forget to factor in the potential savings from prevented repairs and maintenance. A quality fuel additive that effectively treats old gas can help you avoid costly engine repairs down the road. While a premium product might cost more initially, it's often more economical when you consider its ability to protect your engine and maintain ideal performance over time.

FAQs

How Long Can Gas Sit Before It Becomes Too Old to Use?

You shouldn't let gas sit longer than 3-6 months, as it'll start degrading and losing volatility. If you've stored it in a sealed container and added stabilizer, you'll get up to 12 months.

Can Fuel Additives Damage Oxygen Sensors in Modern Vehicles?

If you're using quality fuel additives as directed, they shouldn't damage your oxygen sensors. However, low-quality or excessive amounts of additives can leave deposits that interfere with sensor function over time.

Should I Add Fuel Stabilizer Before or After Filling the Tank?

You'll want to add fuel stabilizer first, then fill your tank. This allows the stabilizer to mix thoroughly with the fresh fuel as it enters the tank, ensuring better distribution throughout the system.

Will Using Multiple Fuel Additives Together Cause Any Negative Effects?

You shouldn't mix multiple fuel additives together, as they can interact negatively and damage your engine. Instead, choose one product that addresses your specific needs to avoid potential chemical reactions or complications.

At What Temperature Should Fuel Additives Be Stored for Maximum Effectiveness?

You'll want to store your fuel additives at room temperature (60-80°F) in a dry place. Don't keep them in extreme heat or cold, and make sure they're tightly sealed to maintain their effectiveness.

Final Thoughts

Just as you're reading this, thousands of engines are struggling with old gas issues. You'll find it's no coincidence that the top seven fuel additives we've discussed align perfectly with today's most common fuel-related problems. Whether you choose STA-BIL's proven formula or Star Tron's innovative enzyme treatment, you're taking an essential step in protecting your engine. Don't wait until it's too late – your engine's longevity depends on it.