All petroleum fuels have a limited storage life. Over time, their quality will degrade and change ("break down") primarily because of chemical reactions that happen in the fuel. These reactions are inevitable and can happen faster if the fuel is exposed, over time, to things like air, water, and catalytic metals. All the stuff that happens in the real world.
Quick Answer
Whether old gas is usable depends on how far it has degraded. If ethanol gasoline has phase-separated into two layers, it is not worth salvaging. If it has only darkened without separating, you can often still burn it—as long as you add a high-detergent fuel additive to control the deposits that form during combustion.
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These aren't the kind of things you can see in real-time, but you see the results over time—gas that has darkened and stratified, and probably undergone phase separation (separation of the alcohol from the gas in ethanol fuels). Diesel fuel has the same problems except for the phase separation bit.
When it comes to dealing with "old gas," that's largely a consumer problem. Very few businesses store gasoline for sizable periods. If it's stored business fuel for any substantial length of time, it's most likely going to be diesel fuel. So that leaves consumers who face the issue. They often buy smaller volumes of gas, put it in containers for some present-day or near-future use, and then forget about it. They find it sometime later, and it's now "old gas."
Sometimes—it depends entirely on how far gone the fuel is. The first thing to check, if it is ethanol gasoline (which most gasoline is), is whether it has phase-separated. If you didn't treat the gas before you put it in storage, after more than a few months it most likely is undergoing phase separation. You'll have two layers in the tank—a layer of water plus alcohol on the bottom, and a layer of gasoline on top.
Fuel that has separated is effectively done. Fuel that has only darkened, but is still one uniform layer, can usually still be used if you treat it correctly. Everything below comes down to telling those two situations apart.
Gasoline goes bad because of chemical reactions that begin the moment it leaves the refinery and speed up with exposure to air, water, and metals. The lighter, most volatile components evaporate first, which is why old gas often smells stale and struggles to ignite. At the same time, the heavier components react with oxygen and start to polymerize—linking together into gums and varnishes.
Ethanol blends add a second problem. Ethanol attracts and holds water, and once enough water collects, the alcohol-water mixture drops out of the gasoline entirely. That is phase separation, and it is the failure mode most likely to make stored ethanol gas unusable.
Look for two telltale signs: separation and color. Phase-separated ethanol gas shows two distinct layers in the container—a cloudy water-and-alcohol layer on the bottom and gasoline on top. If you see that, the fuel has failed and should not go into an engine.
If there is no separation but the gas looks noticeably darkened compared to fresh fuel, that color change means it has reacted internally to the point where some of its heavier-end components have polymerized into gums and varnishes. Darkened gas can still be burned, but those heavier components won't fully combust, and they leave deposits in combustion areas of the engine like injectors and carburetors.
For all practical purposes, no. If the gas has phase-separated, it's highly unlikely you'll be able to do anything worthwhile with it. There are some professional-grade treatments that, used the right way, can help put phase-separated fuel back together. But they're designed to be used by professionals with the know-how and equipment to use them correctly. Those aren't likely to be the options you'll be faced with.
There are bogus fuel additives out there that claim to "restore" phase-separated gasoline. What they really mean is that they "might" help gas that has barely started the process. That isn't what most people think when they decide to try one. They're hoping it will magically repair fully-separated gasoline and make it just like it was before. For gasoline as old as you're likely to find, there's nothing cost-effective that will restore it to its previous homogenous condition.
You can usually still burn it—if you add a fuel additive that contains a high level of detergency. Darkened gas has reacted internally to the point where some of its heavier components have polymerized and are forming gums and varnishes. Run it as-is and those deposits build up in your injectors and carburetor, dragging down engine efficiency.
A high-detergent additive works to help remove these deposits as they're being formed, giving your engine its best chance of staying problem-free while the older fuel burns off. In most cases, burning untreated darkened gas wouldn't be recommended. Treated, it becomes a manageable situation rather than a repair bill.
Both work—the key is choosing a real detergent chemistry rather than a vague marketing claim. If you're in a position where you really have to use old gas, reach for a high-detergent fuel additive. The reputable ones tell you plainly that they contain injector detergents. They won't lean on some generalized "cleaning" claim treated as a secondary benefit of a weird-sounding "new technology." You might see a no-name additive claim it "uses nanotechnology to clean the engine"—c'mon man, seriously?
For standard treatment, use 1 oz. of Mix-I-Go for every 10 gallons of gasoline. For initial treatment, use a double dosage to help remove deposit build-up.
There are plenty of excellent and effective detergent packages out there. PIB detergents (polyisobutylene) and PEA polyether amine detergents are both formulated to be compatible with gasoline that contains more than 20% ethanol. Using a fuel additive built on a higher level of one of those chemistries gives you your best chance of successfully using old gas—provided it hasn't undergone phase separation, of course.
Not in any practical, cost-effective way. Once ethanol gasoline phase-separates, no consumer additive will put it back together. Gas that has only darkened can sometimes still be burned with a high-detergent additive, but you cannot reverse the chemical reactions that already changed the fuel.
Only if it has not phase-separated. Darkened gas that is still one uniform layer can usually be burned, ideally with a high-detergent fuel additive to control the deposits that form during combustion. If you see two layers in the container, do not run it through an engine.
Untreated gasoline often begins degrading within a few months, and ethanol blends tend to break down faster because the alcohol attracts water. Exposure to air, moisture, and metals speeds the process. Treating fuel before storage and keeping containers sealed extends how long it stays usable.
Most consumer products making that claim do not deliver what people expect. At best they may help fuel that has barely started separating. They will not magically rejoin fully phase-separated gas. Professional-grade treatments exist, but they require the equipment and know-how of trained operators.
Both are proven detergent chemistries used in quality fuel additives. PIB stands for polyisobutylene and PEA stands for polyether amine. PEA detergents are often formulated to stay compatible with gasoline containing more than 20% ethanol, which matters when you are treating older ethanol fuel.
You can dilute mildly darkened gas with fresh fuel to use it up gradually, as long as it has not phase-separated. Adding a high-detergent additive helps keep deposits in check while the older fuel burns off. Never blend in gas that has separated into layers.
Treat the fuel with a quality gasoline additive before you store it, keep containers tightly sealed to limit air and moisture, and store them away from heat. For ethanol gasoline, a treatment that guards against water pickup and oxidation is the single best step to take up front.