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My vehicle drinks Oil nearly as quickly as fuel.

The manufacturer recommends changing oil every 10k miles. The vehicle needs oil fill-up every 3-5k miles.

By 10k miles, with 2-3 new oil top-ups, isn't the oil that is being changed the new-ish oil?

Thank you!

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  • Welcome to the site. This is really 2 questions and should be asked separately. Please edit it to remove Q2 and ask it as a separate question. When you ask it please add more detail: are you losing coolant? Do you find pools of oil under your car where it's been parked, etc.
    – GdD
    Aug 3, 2022 at 11:20
  • I've had to add a quart every fill-up and at a midpoint between. (The car had worn valve seals and I was running it rather fast because the freeways were signed for 80-85 mph, and I didn't want to get run over. Poor thing was designed when 55 was the law, but it handles it fine other than that.) Aug 4, 2022 at 16:43

1 Answer 1

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Most vehicles have numerous liters of oil. For example: 4 liters minimum mark, 1 liter difference, 5 liter maximum mark. If you have originally 5 liters of oil, and then:

  • After 3 months, do first top up from 4 liters to 5 liters: 20% new, 80% 3 months old
  • After 6 months, do second top up: 20% new, 16% 3 months old, 64% 6 months old

Now at two top ups, you still have 64% original unchanged oil. Let's see after third top up:

  • After 9 months, do third top up: 20% new, 16% 3 months old, 12.8% 6 months old, 51.2% 9 months old

So after third top up, more than half of your oil is still the original unchanged oil. And you still think that oil change isn't necessary? Think again.

Also oil isn't the only part that requires replacing. Oil filter also does. As a matter of fact, the only reason cars can have long oil change intervals is the oil filter. Splash lubricated small engines like on lawnmowers with no oil filter usually have 50-100 hours oil change interval. Cars, 10 000 miles at 25 mph average is 400 hours oil change interval.

In theory, if your engine drinks oil at a massive rate, let's say over 1.5 liter per 1000 miles, then you could theoretically only change the oil filter without changing the oil at 10000 miles since only 3.5% of oil at 10000 miles would be the original unchanged oil. However, you'll lose the manufacturer's warranty if doing so, and also any goodwill repairs past warranty get probably lost too, so I won't recommend that unless the car's already 10 years old or older.

This "change filter without oil" strategy also works only if whatever impurities that don't get burned with oil get caught in the filter. If there's a form of impurity that's neither caught by the filter nor burned in the engine, it could stay in the oil, so even though only 3.5% of your oil is the original unchanged oil, it can have impurities from the full 10000 miles. A possible impurity in cars used for short trips is gasoline, another impurity for short trip cars is water. Changing the oil every year would remove that gasoline and water from the oil when the oil is drained.

So, long story short: you do need to change the oil filter. I heavily recommend changing oil with filter too even if your engine is drinking lots of oil, so much that it makes oil changes look pointless.

As for oil leaving the engine, that's a different question and usually you should ask only one question at a time, but I'll be short: it's usually burned in the cylinder and turns into white smoke, but could also leak -- if it's leaking you see the evidence of leaks.

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  • You state "the only reason cars can have long oil change intervals is the oil filter" ... this really isn't what allows it. Most vehicles with longer change intervals have more oil in them. While the filter is a factor in this, the amount of oil is the larger factor. Also, I don't see that you've mentioned anything about warranty. If a person only changes the filter, they are voiding the part of the warranty which pertains to the engine. Aug 3, 2022 at 10:10
  • Well I did mention losing the warranty -- if you read my answer carefully, that is.
    – juhist
    Aug 3, 2022 at 10:26
  • ...also if a lawnmower has 0.5 liters of oil per 140 cc of engine displacement, and a car has 5 liters of oil per 2500 cc of engine displacement, I wouldn't exactly say that the car has lots of oil in it.
    – juhist
    Aug 3, 2022 at 11:05
  • Most of the vehicles which have longer intervals (ie: 12-15k miles) have a LOT more oil than 5L ... more along the lines of 8-10L (if not more). Semi trucks with large displacement diesel engines can have 25-30+ gallons of oil in them. This and multiple & larger filters allows them go many, many more miles than you'd see in a car with only 5L of oil. Aug 3, 2022 at 13:21
  • And sorry, yes I see the warranty portion now ... my apologies. Aug 3, 2022 at 13:22

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