Oil looks dirtier when cold/cleaner when hot?

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I'm sorry if this topic has been discussed before, but I've noticed that oil seems to look blackish if checked when cold, and when the car is at operating temp., it tends to return to it's goldish state or at least a dark brown even after 3 or 4K... Can one of you industry guys explain this? I suspect that the unwanted deposits and bi-products of combustion are suspended in the oil when cold, then sink to the bottom of the sump as the oil is warmed? Is this why every manual I've ever read tells me to check my oil when the car is hot, and been turned off for at least 10-minutes? Is this also why oil should only be changed when warmed?
 
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I notice the same effect with the German Castrol I use in both of our vehicles. I have no idea why this is, but I don't worry much about it either. Maybe it's just sitting thicker on the stick when it's cold. Slightly thicker coat of oil on the stick perhaps absorbing slightly more light making it look darker? Just a theory.
 
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just thicker when cold heavier oil film but that said I have seen pure base stock look dark when cold in a blend tank and much lighter when hot >140F. bruce
 
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You check hot because the volume of oil changes with temp. Hot stuff expands. Notice the drip that drops from the dipstick. It will cling longer and be larger when the oil is cold. It is thicker. You are actually looking through more oil when cold.
 
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Like bruce said, the oil is thicker on the dipstick when cold, and of course would be darker than the thin film of hot oil.
 
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