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Interesting question about motor oil.

A

Anonymous-0

Well-known Member
I was asked a question concerning changing of motor oil. When is a multi-grade motor oil, such as 10W-40, thinnest? The way I understand the properties of multi-grade oils is that a 10W-40 oil is 10 weight when cold and 40 weight when hot. Am I correct? This person was wondering if the oil would flow faster when cold, such as changing the oil after then engine had cooled down as opposed to changing hot oil.
 
cool question,next question,10 weight when its cold, 40 weight at what temp? do both flow the same at engine temp?
 
I change my oil when the engine is at operating temperature,HOTTTTTT! The contaminates are in suspension and will flow out with the oil. I also let it sit there at least thirty minutes with the plug out.
 
Single weight oils, say 10W has a certain viscositiy (in centistokes) when cold and is considerably thinner when hot.

40 is a lot thicker (flows slower) when cold, but also is considerably thinner when hot.

10W-40 has the viscosity of 10W when cold and 40 when hot, which means it changes less than either straight weight oil. I do have a book that gives those details, but if you google the oil maker's specifications for a particular oil you can find that data on line.

In summary, 10W-40 is thinner when hot, but not nearly as thin as 10W would be at the same temperture and is thicker when cold, but not nearly as thick as 40 would be cold.

Gerald J.
 
I'm no expert... but as I understand it, multi-viscosity oils are simply formulated from a base weight (say 10W) and then viscosity improvers are added to cause that 10W oil to PROTECT as if it were a 40W oil, but it's still a 10W.
It will always be thinner when it's hot.
Always drain the oil when it's hot unless some circumstance prevents that. This will suspend an contaminants and moisture in the oil so that it will be drained out rather than allowed to pool in the bottom of the pan.

Rod
 
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