You’ve probably got a jerry can sitting in the back of your shed. Maybe it’s for the lawnmower, or perhaps you’re a "just in case" person who likes to have a few liters of fuel ready for a power cut or a fuel strike. But here is the thing: petrol isn't like canned beans. It doesn't sit there perfectly preserved for years. In fact, petrol starts to die almost the moment it leaves the refinery.
Honestly, most people assume that because it’s a chemical product, it’s stable. It isn't. Modern fuel is a cocktail of volatile organic compounds, and those compounds are constantly trying to escape or transform into something that’ll gum up your engine. If you're wondering how long can you store petrol, the short, slightly annoying answer is: not as long as you think. Usually, you’re looking at about three to six months for standard unleaded. After that, you’re basically playing Russian Roulette with your fuel injectors.
Why Petrol Actually Decays
Petrol isn't just one thing. It is a blend of hundreds of different hydrocarbons. When you let it sit, the "light ends"—the highly volatile chemicals that help your engine start when it's cold—simply evaporate. This happens even in sealed containers, though much slower. What’s left behind is a heavier, "stale" liquid that doesn't ignite easily.
Then you’ve got oxidation. When petrol hits oxygen, it reacts. It starts to form gums and varnishes. You’ve probably seen this if you’ve ever pulled apart an old carburetor—that sticky, yellowish gunk that looks like dried honey? That’s what happens when petrol gets old. It clogs the tiny passages in your fuel system. According to the fuel experts at BP and Shell, this process is inevitable, though the speed depends entirely on how you store it.
But the real killer in modern petrol is ethanol.
Most petrol sold today, like E10, contains up to 10% ethanol. Ethanol is "hygroscopic." That is a fancy way of saying it loves water. It literally sucks moisture out of the air. Once the water content in your fuel hits a certain threshold, something called "phase separation" happens. The ethanol and water mix together and sink to the bottom of the tank. This creates a gloopy, acidic layer that can corrode your fuel tank and destroy your engine if it gets sucked up into the combustion chamber.
The Reality of the Three-Month Window
If you are using standard E10 fuel, you should really try to use it within 12 weeks. If you’ve got it stored in a high-quality, airtight metal jerry can in a cool, dark place, you might stretch that to six months. But 12 weeks is the "safe" zone.
Is it going to explode your car if it's 13 weeks old? No. But you’ll notice the difference. The engine might turn over a few more times before starting. It might idle roughly. You might get worse fuel economy.
Different Fuels Have Different Lifespans
It’s worth noting that not all petrol is created equal. Premium fuels (like Shell V-Power or BP Ultimate) often have better additive packages. These additives include antioxidants that can slow down the "going off" process.
- Standard Unleaded (E10): 3 months max. It’s got the most ethanol, so it attracts the most water.
- Premium Unleaded (E5): 6 to 12 months. Because it has less ethanol (usually 5% or less, depending on where you live), it stays stable for longer.
- Storage/Stabilized Fuel: Up to 2 years. If you use a dedicated fuel stabilizer, you’re basically putting the petrol in a chemical coma.
How to Tell if Your Petrol is Stale
Don't just pour it in and hope for the best. Take a look at it. If you can, pour a small amount into a clear glass container.
Fresh petrol should be clear and yellowish, maybe a bit greenish depending on the brand. If it looks dark, like iced tea or orange juice, it’s oxidized. If it smells "sour" or like old paint thinner rather than that sharp, familiar petrol smell, it’s gone. Throw it away. Or rather, dispose of it at a proper chemical waste site. Don't pour it down the drain. Seriously.
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Another sign is cloudiness. If the fuel looks milky, that’s water. That is the phase separation we talked about. If you see two distinct layers in the liquid, the bottom layer is the water-ethanol mix. If you put that in your car, you’re going to have a very expensive bill from your mechanic.
The Best Ways to Keep Fuel Fresh
If you absolutely must store fuel for the long haul, you have to be smart about it.
First, get a metal jerry can. Plastic containers are okay for short-term transport, but they are actually slightly porous. Over months, oxygen can get in and vapours can get out through the plastic itself. Metal doesn't do that. Plus, metal cans are better at keeping the temperature stable.
Keep it full. If you have a 20-liter can and you only put 10 liters in it, the other 10 liters is air. That air contains oxygen and moisture. The more air in the can, the faster the fuel degrades. Fill it to the "fill line" to minimize the "headspace."
Temperature matters. A lot. If your shed hits 40°C in the summer, your fuel is cooking. Heat accelerates every chemical reaction, including oxidation. Keep your cans on a concrete floor in the coolest part of your garage.
The Role of Fuel Stabilizers
If you’re storing a classic car or a lawnmower over winter, you need a stabilizer. Brands like STA-BIL or Lucas Fuel Stabilizer work by creating a protective layer at the molecular level. They stop the oxidation process and prevent the ethanol from bonding with water.
You have to add the stabilizer to fresh fuel. It can't "fix" fuel that has already gone bad. It’s a preservative, not a medicine. Pour it in, run the engine for five minutes to make sure the treated fuel gets into the fuel lines and injectors, and then you’re good for about a year.
What Happens if You Use Bad Fuel?
It depends on how "bad" it is.
If it’s just slightly old, the car might "knock" or "ping." This is pre-ignition, where the fuel burns unevenly. It’s not great, but a fresh tank of high-octane fuel usually clears it up.
If it’s really old, it’ll gum up the fuel pump. It can also cause the fuel filter to clog instantly. In modern direct-injection engines, the tolerances are so tight that even a tiny bit of varnish can ruin an injector that costs hundreds of dollars to replace.
The biggest risk is the acidity. Old petrol turns acidic, which eats away at rubber seals and fuel lines. In older cars, this leads to leaks. In newer cars, it leads to sensor failures.
Practical Steps for Long-Term Storage
Stop treating petrol like a "set and forget" resource. It is a perishable product.
- Rotate your stock. If you keep 20 liters for emergencies, pour that 20 liters into your car every two months and refill the can with fresh stuff. This ensures you never have "dead" fuel when you actually need it.
- Label your cans. Use a Sharpie and a piece of tape. Write the date you bought the fuel. You think you’ll remember. You won't.
- Buy Premium for storage. Even if your car runs fine on E10, buy the E5 or "Super" unleaded for your storage cans. The lower ethanol content gives you a much bigger safety margin.
- Drain your small engines. If you aren't going to use your lawnmower or pressure washer for more than a month, drain the tank or run it until it dies. The tiny carburetors on these machines are the first things to fail when fuel goes off.
Actionable Next Steps
Check your shed right now. If you have a jerry can and you can't remember if you bought the fuel before or after the last holiday season, it’s probably time to use it up.
If the fuel is less than six months old, the safest bet is to dilute it. Pour it into your car's fuel tank, but only if the tank is at least three-quarters full of fresh, high-quality petrol. This dilutes the "stale" chemicals and the engine will likely burn it off without any drama.
However, if that fuel has been sitting for over a year, do not put it in your car. It’s not worth the risk of a $1,000 repair bill just to save $30 worth of petrol. Take it to a local hazardous waste center. Most councils have a day where you can drop off old oils and fuels for free.
Moving forward, adopt a "three-month rotation" rule. It’s the only way to be 100% sure that when you turn the key in an emergency, the engine actually roars to life.