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My car manual says that oil level should be checked with a warm engine (to account for thermal expansion) after letting the car sit on a level surface for 5 minutes after turning off the engine (to allow oil to drip down back to the oil pan).

I recently followed this procedure. I pulled the dipstick, wiped it clean of oil, pushed it back to the engine, pulled it and observed the oil level. It's actually very hard to define where the "oil level" was.

My observation was the following:

  • The dipstick was covered by a thick layer of oil to about half the level between "low" and "high" marks
  • From about half the level between "low" and "high" marks, the layer of oil gradually became thinner and thinner. It completely vanished somewhat above the "high" mark.

The dipstick isn't completely flat, it has two protrusions, one way below the "low" mark and one way above the "high" mark. This ensures that when you push the dipstick into the engine, the gauge on the dipstick never touches any surface inside the engine, but rather only touches the oil. So it should be getting the oil only from the oil pan, not from the dipstick tube if the dipstick tube surfaces are covered with oil.

It is (and has always been) clear to me this reading means there isn't too little oil in this engine. The car has always been this way, so it doesn't appear to consume any oil.

However, I'm a bit mystified by why the reading on the dipstick is so vague and hard to define.

I could argue that since the oil on the dipstick only ends somewhat above the "high" mark, that the engine is slightly overfilled with oil, and the oil was thicker at the lower parts of the dipstick because when the dipstick is pulled from the engine, the oil starts to slowly run down the dipstick.

I could also argue that since the oil layer on the dipstick gradually begins to become thinner halfway between the "high" and "low" marks, that the engine oil level is halfway in the range where it should be, and the thin layer of oil even above the "high" mark was caused by capillary action causing the oil to run up the dipstick (but would the capillary effect in this case be so large?).

When checking the engine oil level when cold after the car has been sitting overnight, the ambiguity seems to be far lower (perhaps due to higher viscosity when cold?), but then there are two effects causing measurement errors: (1) the engine oil is cold, affecting the reading due to thermal expansion, (2) the engine oil has had more than 5 minutes to drip to the oil pan, affecting the reading in a direction opposite to the thermal expansion. I usually have previously checked the oil level after the car has been sitting overnight and the engine is cold, but recently I became aware this may be an incorrect way to check oil level and then decided to check the oil level when the engine is hot and has been sitting for 5 minutes to allow oil to drip back.

But which of these two arguments would be correct? There's 1.5 liters of difference between "low" and "high" so the ambiguity in the dipstick reading amounts to approximately 0.9 liters.

The car is 2016 Toyota RAV4 hybrid with 2AR-FXE engine.

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  • Capillary action would not be occurring. Capillary action happens through absorption. If anything, the physics term which might be happening is called "wetting", which is where a fluid is drawn up a solid object due to its makeup. You can most noticeably see this effect in how water will edge up the walls of a glass container. Also, I'm not sure I understand exactly what your question is? Which two arguments are you referring to? Apr 18 at 14:12
  • A photo of what you're seeing might be really helpful. I have noticed that some dipsticks can be hard to read.
    – jwh20
    Apr 18 at 14:18
  • If you first removed, wiped and replaced the dipstick before reading it again, the first pull may have dragged a little oil up the tube (from the bottom of the stick) which was then running back down when you pulled it the second time. But it seems clear that you could still see what the oil level is. Next time, look at the stick the first time you pull it, and then wipe, replace and pull it again. Apr 18 at 17:36
  • @WeatherVane - Depending on how its put together, it might do that every time you stick it to get a reading. Can be a complete PITB. Apr 18 at 22:46

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Whenever the oil level is not obvious, your guide should be how low is the dry area on the dipstick. Your true oil level will never be higher than the lowest dry spot on the dipstick. Because that would be impossible, unless your oil had big bubbles. So look for the lowest dry spot.

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