If you’ve got a car and either a garage or shed, the chances are you’ve got some old engine oil kicking about. Likely it’s nothing more than a 1 litre top up bottle, given to you at your last car service, or maybe the one before, or the one before that. Maybe it’s a 4 litre can you bought to do that between service oil change you never got round to doing. You’re not sure how long it has been there so the question is – does motor oil expire or can I still use it?
What Do The Oil Companies Say?
There is a general consensus of terminology coming from the oil companies that unused, unopened and stored in its original container out of extremes of temperature, motor oil will last for an “extended period”. They then go on to suggest that the oil shouldn’t be used after a few years; the exact period varying between 2 years (according to Total) up to 5 years (Mobil).
So whilst motor oil manufacturers say we should responsibly dispose of oil after a few years they all stop short of saying that the oil does actually expire. Clearly there is a vested interest in their future sales here so to get to the truth about oil we need to drill down deeper.
Why Do We Need Motor Oil?
The primary purpose of motor oil is for lubrication. Inside the internal combustion engine, hard metal surfaces slide backwards and forwards across each other at high speed and at very high temperatures. The oil forms a fluid barrier between the moving parts, lowering friction and, crucially, reducing wear on major engine components like pistons, connecting rods, crankshaft and of course the cylinder linings of the block itself.
Secondly, the oil also acts as a coolant, drawing heat away from the engine as it circulates and third it keeps the engine clean by carrying away dirt and debris from high wear areas.
How Does Motor Oil Expire?
The manufacturers’ advice on storage gives some clues as to what might happen to degrade the oil. Extremes of temperature can affect oil in different ways. Too hot and thermal breakdown occurs reducing the oil’s viscosity (its “weight” or thickness), which if used leads to a decreased flow rate and in turn to increased oil consumption, the build up of deposits and potential damage to engine surfaces. Too cold and sludge will form in the oil, blocking engine oil passages and eventually becoming too thick to flow at all.
Another possible degradation cause is oxidation, more likely with a container that’s been opened, which is where the oil molecules react with oxygen molecules in the air and this time viscosity increases forming sludge and sediment. Oxidising oil can also increase in acidity and start to rust or corrode any metal surfaces it contacts.
An opened, part used container also renders the oil more susceptible to contamination from water condensation or dirt, both of which reduce its effectiveness as a lubricant.
Does The Type Of Oil Make A Difference To Whether It Expires?
Simply put, yes, the type of oil matters a lot. Key to this is whether the oil is synthetic and what additive ingredients there are. Both conventional and synthetic oil began life coming out of the ground but synthetic oil is further refined, distilled, purified and broken down to a basic molecular level. From this point the oil’s molecular structure can be rebuilt and customised to suit a specific purpose, such as the greater demands of heat tolerance in modern engines.
Even though this more uniformly engineered base oil is already an improvement over conventional oils, it is further improved through a blend of additive compounds. These extra ingredients counter the typical breakdown causes maintaining viscosity at extremes of temperature through detergents and dispersants which delay sludge build up, isolating contaminants by suspending them in the oil and counteracting the effects of oxidation.
What About Oil Inside An Engine, What Happens There?
Everything that could happen to break down oil in a container, is even more likely in use inside an engine. It is subjected to much higher temperatures and, of course, mechanical action. As conventional oils in use start to break down straight away, the maximum recommended driving between oil changes used to be only a few thousand miles. Depending on whether you drive hard miles – characterised by lots of short trips where the engine oil barely warms up – or easy miles – such as longer highway journeys – your engine oil can last a full-service interval such as 18,000 miles.
The biggest issue in use now is contamination. As the oil circulates around the engine it picks up dirt such as soot and microscopic slivers of metal worn away from the moving parts. This is good in that it takes the dirt away from the engine but even with a good oil filter it can eventually become saturated with debris and start to form a sludge.
What Do We Recommend?
Only put into your engine the type and grade of motor oil recommended by the car’s manufacturer. Brand is less of an issue but it’s probably best not to mix brands as they’ll have different additive recipes which may not work as effectively together. Make sure the oil and oil filter is changed according to the mileage intervals given in the owner’s manual or if the car has been sat unused for more than a few months.
Whilst premium brand fully synthetic motor oils can be quite expensive and you might be reluctant to waste it, the cost is trivial compared to having major surgery on your engine to replace worn components caused by filling up or topping off with degraded oil. So does motor oil expire? Yes it can but depending on the conditions and the type of oil it might take a long time. Unless there is an expiry date on the container and it’s still in the future or you know exactly how long you’ve had it and how it’s been stored, it’s time to take those cans, part-cans and bottles of motor oil down to your local recycling centre and remove the temptation to use it.