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0-20w oil is straight 20 weight oil
20 weight is when it is hot and when its cold
oil like 5w-20 is a multigrade oil (2 weights, 5 weight and 20 weight)
modern oils have additives and polymers to reverse the laws of physics
in a normal liquid, the molecules expand away from each other when it is hot
motor oil, however, contract at higher temperatures to create a higher viscosity at operating temperature
when it is 195 degrees Fahrenheit in your engine, your oil's viscosity is 20 weight
but when it is cold, it is 5 weight
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