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How Long Can Synthetic Oil Sit In an Engine? – Surprising Answer

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With the apocalyptic status of the last and the current year, an increased number of cars and motorcycles have been left sitting untouched for a long time. 

Naturally, vehicles don’t like sitting around for extended periods of time as they are made to be driven regularly. Over time, most materials inside the car start to degrade, rust, and even the engine itself becomes more reluctant to start.

Now, if your car or motorcycle has been parked for a long time, you might be tempted to change the old oil out because it might have gone bad.

But is it really necessary? Does engine oil even go bad after sitting unused? And how long can oil sit around before it becomes too old?

Does Oil Go Bad After Sitting Unused Inside an Engine?

The short answer is yes, oil does go bad after sitting unused inside an engine – at least according to the manufacturer. Believe it or not, even unopened, perfectly sealed engine oil inside jugs does have a shelf life – 5 years to be exact.

Whether that’s true or just a way for oil manufacturers to squeeze more purchases is difficult to answer, however, engine oil already is at least 10 million years old, and I doubt that a few more years here and there inside a sealed container will do any harm.

Four jugs of brand new engine oil

However, unlike sealed oil jugs, the inside of a car’s engine contains acidic contaminants and moisture, which is one of the few reasons why engine oil degrades and needs to be changed over time.

Even when your car is sitting unused, due to temperature and atmosphere changes, moisture will still build up inside the engine and drip down to the oil pan.

Moisture Contamination Within Engine Oil

Unfortunately, moisture is the second most destructive contaminant of engine oils, with the first being particle contamination. Moisture contamination can be categorized into Dissolved, Emulsified, and Free Moisture – in this case, we’re mostly dealing with Free Moisture.

Free moisture occurs from condensation and is not dissolved within engine oil. It is denser than oil, thus it settles down to the bottom of the engine and coats metal components with a thin film. Normally, free moisture evaporates due to heat when the engine is running at operating temperature, however, if a car is left sitting for a long time, free moisture does not get burned off and starts to rust metal parts.

Abandoned car sitting unused in a field

Moreover, once the engine is running after a long time, that undissolved moisture will dissolve within the engine oil and chemically react with metal surfaces, eventually degrading metal surfaces and weakening them.

Now, the effects of moisture contamination vary greatly according to local climate, temperature, ambient humidity, and the amount of time the vehicle has been sitting unused. However, unless your car has been stored for 5+ years, the effects of moisture contamination are very minimal.

Lastly, apart from moisture contamination, engine oils also lose their properties over time due to reacting with oxygen. The additives inside the oil, detergents especially, are prone to oxidation and become more acidic (corrosive) over time.

How Long Can Synthetic Oil Sit in an Engine?

The exact amount of time that engine oil, especially synthetic oil, can sit around unused inside an engine before it loses most of its properties varies greatly depending on a lot of factors – local climate, temperature, ambient humidity, additive package, oil type, amount of time the vehicle has been sitting unused, etc.

For example, if you’re living in a dry state with extremely low humidity like Nevada, then the oil inside your car’s engine can sit for a much longer time when compared to a high humidity state. Because the ambient humidity is low all year round, there are fewer chances of moisture condensing inside the engine.

But to give you a rough idea overall, in most cases, synthetic oil can sit inside an engine unused for a maximum of 2 years before it loses its properties and needs to be changed (although it’s recommended to change it after a year). As for blends and conventional engine oil, that timeframe is reduced to 1-1.5 years, as mineral oil is more susceptible to oxidation and hydrolysis.

If you’re living in a dry environment of a desert, then those time frames can be extended by a year or two.


Nowadays, more and more vehicles are left sitting inside the garage. All that sitting around takes a toll on the rubber components and the fluids inside the engine – engine oil included.

Luckily, synthetic engine oil degrades quite slowly and should be fine to use before it reaches the 2-year mark of sitting inside the engine unused. Conventional oil, on the other hand, is more susceptible to moisture and oxidation, there it should be changed at the 1.5-year mark of sitting around.   


  1. Know the Effects of Water ContaminationNoria
  2. Moisture in Oils: The Three-Headed BeastBrookfield 

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