Synthetic oil can go bad over time, even when just sitting unused in an engine or storage container. However, synthetic oils are formulated to be more resistant to breakdown than conventional oils, so they have a longer shelf life. Here’s a look at how synthetic oil can go bad when not in use, how long it generally lasts, and tips for prolonging its life.
Can synthetic oil go bad when not used?
Yes, synthetic oil can eventually go bad or lose effectiveness when it is not being used in an engine. Here are some of the main ways synthetic oil can deteriorate over time:
- Oxidation – Exposure to oxygen can cause the oil’s molecules to break down. This leads to increased viscosity, sludge buildup, and acid formation.
- Thermal breakdown – Prolonged exposure to high temperatures, even when not in use, can accelerate the oil’s natural thermal degradation.
- Moisture contamination – Absorbing moisture from the air can dilute additives and cause corrosion in the oil.
- Light degradation – UV rays from sunlight can react with the oil molecules and compromise their integrity.
- Additive fallout – Key additives that give synthetic oil its enhanced properties can separate and precipitate over very long periods of non-use.
So in summary, synthetic oils are still vulnerable to natural chemical breakdown processes like oxidation, thermal cracking, and additive depletion even when not in active use. But due to their specialized synthetic base oils and additives, the degradation happens much slower compared to conventional oils.
How long does synthetic oil last in storage?
Synthetic motor oils stored properly can generally last around 5 years before significant degradation occurs. However, some factors affect how long synthetic oil will last in storage:
- Storage container – Stored in an airtight, opaque container out of sunlight is ideal. Exposure to oxygen, sunlight, or moisture will accelerate breakdown.
- Storage temperature – Cool, consistent temperatures like a climate-controlled warehouse will prolong life vs. hot garages or sheds.
- Oil brand and type – Fully synthetic oils (like those from Amsoil, Mobil 1, Royal Purple, etc.) last longer than synthetic blends.
- Antioxidants – Higher quality synthetics have more antioxidants to resist oxidation.
Most synthetic motor oils have a shelf life of about 5 years from the manufacturing date if stored properly in unopened containers. Some premium synthetics claim to last 8-10 years in storage due to extra additives and antioxidant packages. So you can safely store synthetic oil for car or motorcycle engines for around 5 years before it goes bad.
Does synthetic engine oil expire?
Like conventional oils, synthetic engine oils do have expiration dates printed on the containers, which are usually 5-6 years from the manufacturing date. However, synthetic oil does not necessarily “expire” after that date or stop working. The printed expiration date is more of a general guideline for the oil’s usable life.
High quality synthetic oils may only show minor viscosity changes and retain adequate additive levels for 7-8 years or longer when stored properly. Lower grade synthetics may degrade faster. So while you can technically use synthetic oil past the printed expiration date, it’s best to follow the manufacturer’s recommendations for maximum performance.
Signs that synthetic oil has gone bad
Here are some signs that the synthetic oil sitting in your garage or storage may have gone bad and needs to be replaced:
- Very dark color – Has oxidized and chemically broken down
- Sludge or deposits – Contaminants and degradation byproducts
- Thick, viscous texture – Indicates oxidized oil molecules
- Low viscosity – Can mean diluted oil or additive fallout
- Acrid or burnt smell – Oxidation causes acidic compounds
If synthetic oil shows any of those qualities, it should not be used in an engine anymore. While premium synthetics have good storage life, it’s always best to check the oil periodically when storing it long term.
How to prolong synthetic oil shelf life
Here are some tips to get the maximum shelf life out of your synthetic engine oil when storing it:
- Buy high quality synthetic oil with enhanced antioxidants like Amsoil Signature Series or Mobil 1 Extended Performance.
- Store oil in a cool, climate controlled location if possible.
- Use unopened oil containers and seal them tightly.
- Keep containers out of direct sunlight and extreme heat.
- Use opaque plastic or metal containers instead of clear ones.
- Avoid storing oil in humid locations where moisture can get in.
- Limit headspace in containers to avoid oxygen contact.
- Never mix used oil with fresh, clean oil in storage.
Following those precautions when storing synthetic oil can maintain its quality and performance for many years. Just be aware that no oil lasts forever, so check your inventory periodically.
Can you use expired synthetic oil?
Technically, you can still use synthetic motor oil even if it is past the printed expiration date, as long as it was stored properly. However, using expired synthetic oil is not recommended for optimal engine protection.
It’s always best to follow the manufacturer’s guidelines for oil change intervals and shelf life recommendations. Using severely degraded or oxidized oil can cause increased engine wear, sludge buildup, corrosion, and clogged filters. Periodically check oils in storage and look for signs of contamination or breakdown before using in an engine.
For the highest level of engine protection, it’s advisable to use fresh, unexpired synthetic oil within the manufacturer’s recommended lifespan. But in a pinch, recently expired synthetic oil that was stored correctly may work temporarily.
Does synthetic oil need to be changed before storage?
It is not absolutely necessary to change your engine’s oil before long term storage if it is a high quality synthetic. Synthetic oils are very stable and can handle sitting unused better than conventional oils. However, consider these benefits of an oil change before storage:
- Removes contaminants and acidic byproducts to minimize corrosion
- Replenishes additives to maintain optimal performance
- Applies fresh oil that will last longer in storage
- Allows switching to synthetic if the current oil is conventional
- Gives peace of mind knowing the engine is protected
Consult your owner’s manual – some manufacturers specify changing oil before any long term storage to remove contaminants. An oil change is affordable insurance to make sure used oil doesn’t go bad prematurely when sitting.
Conclusion
Synthetic engine oils are formulated to withstand long periods of storage without use. But they can still degrade eventually from oxidation, thermal breakdown, and contaminated storage conditions. With proper handling, most synthetics last around 5 years in storage before significant chemical changes occur that reduce performance and protection. Using expired or degraded synthetic oil is never recommended. Storing oil properly and checking it periodically can help maximize the shelf life of synthetic motor oil.
