DOES MOTOR OIL EXPIRE: OPENED OR UNOPENED, HOW LONG DOES IT LAST?

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-liter 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-liter can you bought to do that between oil changes you never got round to doing. It leaves you wondering then… Does motor oil expire? You’re not sure how long it has been there so the question is – does motor oil expire or can I still use it? 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. 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. This time, viscosity increases forming sludge and sediment. Oxidizing oil can also increase acidity and start to rust or corrode any metal surfaces it contacts. Simply put, yes, the type of oil matters a lot. The 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 customized to suit a specific purpose, such as the greater demands of heat tolerance in modern engines. 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 – characterized 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. Motor oil disposal is often ignored by car manufacturers yet is a task many of us will have to carry out. Most companies will advise that you change your car’s oil every 5,000 or so miles, to ensure that the engine runs smoothly and that your car will continue to perform optimally. However, few manufacturers will actually tell you what to do with the dirty oil once it has been changed, and more to the point, how to dispose of it. The type of oil you will most likely dispose of is waste oil such as fuels and lubricants, which are commonly used in the average car engine. The irritating news is that you can’t just put oil in with the trash or pour it down the drain, oil is classified as a highly toxic, hazardous chemical. Clean oil is relatively safe but once it is used in an engine and no longer clean it becomes a carcinogen. The oil company Pennzoil said this regarding oil’s toxicity; ‘Used oils may contain harmful impurities that have accumulated during use