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?
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.
Unfortunately, the answer of "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." is utter BS.
I say this because the next sentence in your owner's manual, that you neglected to mention, is this: "Switch off the engine and wait a few minutes for the engine oil to flow back into the oil pan." That sentence kind of dispels that notion that you're only checking "the oil reserve", whatever that is.
No, you're checking it hot after waiting for a few minutes to allow the oil to drain back into the pan because hot oil expands in volume vs. cold oil. True, the expansion is small, but VW must think it's enough to suggest checking the oil hot instead of cold, after an appropriate wait time to allow all the oil to drain back into the pan.
As your engine sets overnight your oil gets colder and becomes just a bit thicker and does not move as freely as when it is warm. When the engine is warmed up the oil is softened and gives a better more settled reading as to the actual amount of oil in your motor.
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.
