It depends how much exposure it receives to things that can speed up its degradation. As such, how you store the oil influences its shelf life. Your oil will last longer on the shelf than you think it will. If you have motor oil that you have no idea how old it is, just have a visual look at it. Pour it out of the container so you can examine it. Fresh motor oil has a pale brown color to it. If the oil is significantly darkened, it may have oxidized.
If it has a milky appearance, there could be water in it. It should not be hazy, nor should you be able to detect any kind of settling, or the presence of any particulates.
The other question concerns how long your oil is good for in your engine - how long it "lasts" in your car. This is a much different question, by nature, than the first one. But like the first one, this is situation-dependent - except the modifier here is driving conditions instead of storage conditions. Driving conditions are the biggest determiner of how long oil lasts in your engine. To stop the motor oil from freezing at low temperatures, manufacturers add pour point depressants, or PPDs.
PPDs are a kind of polymers that work to inhibit the forming of wax crystals so that the motor oil flows freely. Detergents and dispersants in the motor oil delay the formation of sludge and help keep the oil clean by suspending contaminants in the oil. Oxidation inhibitors help maintain the stability of the motor oil while rust and corrosion inhibitors prevent condensation.
Calcium additives ensure that the engine receives the oxygen it needs. They also help to delay the formation of sludge and varnish. It is advisable to store the motor oil at constant optimum temperatures of between 40 degrees and 85 degrees Fahrenheit. Very low temperatures can result in the formation of sediments in the motor oil. Exposure to humidity can cause condensation in the motor oil. This moisture can react with the additives in the motor oil and form insoluble particles.
Sludge can also form and the motor oil can turn into a breeding ground for microbes. If you have stored the motor oil in a cool, dark place and it has not passed the expiry date, you could use it. However, you must check it carefully for its consistency. If it is thick and sticky, and has solid or gel-like particles, do not use it. If you see that the additives seem to have separated from the oil, it is not safe to use. You must also check its color. Usable motor oils have a clear appearance and if you notice any milkiness or haziness, that is a sign of water contamination.
A change in the original color of the motor oil could also indicate its instability. Despite keeping opened or unopened bottles carefully, changes in the temperature of your storage area can result in the air moving in and out of the bottle.
This is especially true if there is vacant space in it. At high temperatures, air forces out of the container and at low temperatures, air is pulled into it. This can result in the motor oil degenerating over time. If you have recently bought a new car or any other vehicle, check if the motor oil you have is compatible with the engine of the new vehicle. Manufacturers upgrade engines all the time and your old motor oil may not be able to meet the new standards and requirements.
Using the wrong oil can result in damage to your engine. Regulations pertaining to the motor oil being used in cars can also change over time. Make sure your motor oil is compliant with them before using it. But ask any tribologist with an ounce of morals, and they will tell you that modern engine oil is sufficient to last MANY thousands of miles, perhaps K.
I get that you speak hours - I'm too lazy to translate that to knots. And that's good old dinosaur juice. Modern synthetic compounds have wonderful properties, and might last twice as long as the organic stuff. In fact, with the exception of point 2, the right synthetic might last 50, miles. Oil gets dirty.
Little particles of soot and combustion products and small insects and detris and debris and schmutz and water accumulate in your oil storage "area". A really good oil filter [e. Oberg] might filter down to 5 micron or less, but require frequent cleanings. Hence you only see this type of filter in racing and special applications. So what? Some of that dirt is abrasive. None of it is helpful. It is circulated under pressure. Now imagine that your very expensive synthetic oil is circulating little tiny abrasive bits all around those critical lubricated interface points.
Doesn't matter how much the "excipient" [big word] costs, what you have created is a cutting fluid - no matter how expensive.
The size of the little cutter buggers is only limited by your oil filter. And your standard oil filter is a balance between longevity and particle size. Did I yell that yet? Unless you are parked in a desert monsoon or something. That's not even a thing. It just doesn't happen. I know such statements are gonna get me some negative attention.
But to summarize, oil does not "spoil", and non-use certainly doesn't sever the long chain molecules. Yes, moisture that accumulates from combustion by-products should be vaporized off once in a while, but oil is not hygroscopic, and extended storage will not add bad things.
The only issue is "rundown", or long term storage having the oil seep out of critical wear points However, again, this is NOT a function of the oil, oil type, or change frequency. The bottom line is that you will be much better served starting your boat motor every month for a minute or two in drydock. Even without cooling, if your unit draws seawater.
This will "re-wet" the important surfaces with oil and prevent terrible "cold-start" damage.
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