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Can engine oil be used past it's monthly cycle?

If the manufacturer specified the oil/oil filter change interval to be every 5k mi or every 4 months, but it took someone one 1 year 2 months to run the the engine 5k miles, would the engine oil still be good past the 4 month mark? Given average temperatures and climates.

My concern is whether you should change the engine oil when it's been inside the engine for 4 months even if you don't reach the mileage mark (e.g., 4k mi - 5k mi)?

Would the engine oil loose some of its lubrication and protection after 4 months or so?

I usually just go for the 4-5k mi mark but I want to make the right call to keep my engine protected. Specifically, for a 2003 truck over 200k mi using full synthetic oil.

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  • Modern engine oils are phenomenally good and are not degrading over months of normal use. My (more modern) truck specifies oil changes at 10k miles. How many months did the oil spend in the store waiting to be bought?
    – Jon Custer
    Nov 6, 2023 at 17:42
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    @JonCuster - That is a very poor analogy. Once in the engine, oil has contaminates which degrade the oil. These contaminates include unburnt fuel, old oil, and moisture. And then you also have heating cycles. You cannot compare it to something which is just sitting on the shelf. Nov 6, 2023 at 20:24
  • The point remains that today's motor oil, particularly synthetic, is much better than anything 30 years ago. And time sitting does not contribute significantly to degradation.
    – Jon Custer
    Nov 6, 2023 at 21:45
  • @JonCuster well, the oil was used about 4,900 miles over one year and 2 months. I didn't use the truck that much for a few months cus I was unemployed.
    – LVX-001
    Nov 7, 2023 at 5:31
  • @Pᴀᴜʟsᴛᴇʀ2 I do understand that fluctuating temperatures in the climate, old oil, unburnt fuel, etc. contribute to the degradation of the oil. That factor also made me think whether it was okay to use oil past the 4 -5 month mark. My oil has now been inside my engine for about 1 yr 2 months now and it has about 100 mi to go before it hits the 5k mi mark. I do plan on changing it before that though.
    – LVX-001
    Nov 7, 2023 at 5:38

3 Answers 3

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Depends on the type of oil. Modern synthetics have a sufficient number of additives to last half a year under normal conditions without noticeable degradation. However, increased load, temperature changes, low quality fuel and other factors, including time, slowly destroy the structure of the oil and it will no longer have the same properties as new.

If you drive a little, but want your engine to last a long time and at the same time change oil less often, fill in high-quality oil with ester (>+5%).

Esters help create and maintain a strong oil film in the cylinders.

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IMHO, it will be perfectly fine with a decent modern oil. I'm quite surprised to see that they're still suggesting such short intervals (I presume US market vehicle?). My own car (a Diesel, which tend to be harder on oil) is 12 months or 12,000 miles, and that's pretty standard for anything I've seen in the UK in the last 20 years.

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It's important here to understand why oil ages.

In the bottle, oil has a shelf life of maybe 5 years at room temperature. However, you should shake the bottle if you plan on pouring it only partially, since the various constituents are not perfectly mixed after sitting on the shelf for years.

In a car, oil ages primarily due to heat. Note that Arrhenius equation governs oils stability. Generally, every 10 degree Celsius increase cuts the life to half. So at 100 C, the lifetime of oil is probably only 7 days.

Typically, a car is driven at average speeds of maybe 40-50 km/h including time stopped but engine running. This means that a particular kilometer amount translates to particular amount of time the engine oil is hot. So manufacturers generally specify a kilometer amount, let's say to change the oil every 15000 km.

However, some people drive in city centers. Generally, their annual kilometers driven is very low, but the amount of hours the engine is hot is not proportional to highway driving, since:

  1. City center driving is slow, so the average speed is much less
  2. The car doesn't immediately cool down after a trip, so frequent short trips are worse than less frequent long trips, because for the same number of hours driven, the engine oil is hot for a longer amount of time in city center driving, since the trips are short

Because of the different oil temperature characteristics of city center driving, car manufacturers generally say to change the oil every year at the very least.

If your car is under warranty, you should follow the manufacturer's recommendations. But I would feel safe in extending the oil change interval to 2 years from the typical 1 year, given three conditions are met:

  1. The kilometer amount for oil change is not reached in the oil change time
  2. The car is no longer under warranty
  3. The car is driven for long high-speed trips but very rarely so annual kilometers are very low, not city center driving but highway driving

A 4 month oil change interval is probably way too short. Did the car manufacturer really specify that? What is the make/model/year of the car?

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