Reader Dave Bennett recently reached out, asking about the shelf life of some aviation oil he had in his hangar. This is quite common for pilots who have a long rebuild or restoration for their aircraft.
The answer is fairly easy from a legal standpoint because the Military and SAE specifications state that the supplier must guarantee that the product stay within spec for at least three years. So, if you are a commercial operator or lubricant sales organization, the shelf life is three years.
But what if you are a private pilot? What’s the “technically safe” shelf life of aviation oil?
When I was still with Shell Oil, I would routinely inspect the company’s lubricant blend locations. They all were required to retain samples of every batch and keep them in storage for a minimum of three years.
Every once in a while, I would find a really old sample that had been overlooked. If the sample was more than five years old, I would have it sent to the lab and tested.
After letting the sample settle, I would pull a bit off the top and a bit off the bottom and test them.
For straight mineral oils meeting Mil-L-6082/SAE 1966 spec, every sample met the spec and there was no difference between the top and bottom of the sample.
For straight weight AD oils meeting the Mil-L-22851/SAE J-1899, all of the samples met the requirements of the spec with a very small variation in viscosity from top to bottom.
The same is true for Aeroshell Oil W 15W-50. Except for a little variation in viscosity, it was still well within the spec.
In the late 1990s, a gentleman sent me a sample of Aeroshell Oil W100 that was well over 20 years old. The container had some deterioration and the oil, while just meeting the specification, had significant changes from the new spec. I recommended that he take the oil down to a flea market and sell it to collectors. He called back and said he sold the whole lot for over $10 a quart.
The other part of the answer is the shelf life of oil in an opened drum.
This depends on how and where the drums are stored.
A sealed 55 gallon drum that sits outside is not sealed. Even with rain caps, the oil will probably take on some water, because the drum will breathe and take in warm moist air during the day and then the moisture will drop out at night when it cools down.
To dispense oil from the drum, people either put the drum on a rocker or put a drum pump in. Either system pulls water off the bottom and into the oil going into the engine.
I have a drum of oil on my farm that I use in tractors and other equipment. Last year was very wet and the oil became milky. I took a metal container of the oil and heated it to 250°F on a hot plate. In a few minutes, the water boiled off and the oil was clear.
Now I change oil using the milky product and then after changing the oil, I take the machine out and work it hard to get the oil temperature up. After an hour or so I check the oil and it is usually clear and looks normal. If not, I run it some more.
I am a little hesitant to recommend that process to pilots because so many aircraft are used in short flight service and do not get the oil temperature up to around 180°F to boil off the moisture.
I am very much aware that I sound like a broken record about getting oil temperature gauges calibrated and making sure that pilots get the oil temperature up to 180° during cruise.
But the number one thing that pilots need to do to ensure proper engine life is boil off the moisture during operation. Even if you do not start with milky oil, moisture will build up when your plane sits and it must be boiled off or it will start rusting your high-priced engine parts.