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The owner's manual to my VW Golf Mk7 states

With the engine at operating temperature, park the vehicle on a level surface to ensure that the engine oil reading is correct.

Why is it important for the engine to be at operating temperature when checking oil level?

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2 Answers 2

up vote 4 down vote accepted

tl;dr: This procedure checks the oil "reserve" remaining in the pan after all moving parts are lubricated.

This procedure is purely vehicle dependent. For example, on my car, I'm supposed to check the oil when the engine is cold.

In asking you to check when the engine is hot, the oil will not be pooled in the oil pan. Instead, it will have been fully circulated to all the moving parts that badly need that oil to function. As a result, you'll be checking the oil reserve that remains in the pan.

Note, this oil isn't extra. Among its many functions, oil is also carrying some of the heat load of the engine. All those hot moving parts get the oil in contact up to painful temperatures. An "excess" allows the oil to absorb more heat without burning completely away. The oil in my car leaves the block, passes through an oil cooler and then to the oil pan before circulating back into the fray.

Note: I can also check the oil in my car when hot by using different marks on the dipstick. I prefer the cold procedure so I can confirm total oil.

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Thanks! So would I be right in thinking that, if I checked the oil when the engine is cold, the dipstick will effectively over-read? –  Flup Mar 12 at 11:39
@Flup, yes, if you incorrectly used the same marks, you'd think that your car was badly overfilled. I suspect that your car is like mine, though, and there will be other marks on the stick that can be used for a cold vs. hot reading. –  Bob Cross Mar 12 at 11:45
Lovely -- thank you :) –  Flup Mar 12 at 11:46

Most servo attendants will offer to check your oil when engine is hot.Some will even show you the dipstick to validate claim engine needs oil.Oldest trick in book to sell you more oil.It is then overfull causing a smoky exhaust.Always check on level ground when cold to give accurate reading.Conversely auto-trans fluid should be checked on level ground in park or out of gear with engine running.

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How does this correspond to the previous answer which states that checking while hot is mandatory for some vehicles? –  Flup Apr 7 at 10:53

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