I'm stuck on the simple stuff: Which oil to buy?

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Blue99, thanks for posting the viscosity chart. Based on kinematic viscosity alone one might conclude that you could heat the 5W-30 10degF hotter than the 5W-20 and that they would behave the same in the engine. But this is not the case, and is a general misconception here on BITOG.
 
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But this is not the case, and is a general misconception here on BITOG.





No, "Bitog" does not have a problem with basic viscosity theory!
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Viscosity is viscosity and it is not dependant on the composition of the fluid. 40 cSt of honey, corn syrup, vegetable oil or motor oil all measures the same in viscosity test conditions.

The components of a 5W-20 motor oil are a 4-5 cSt base oil plus VI improver plus add pack.

The Components of a 5W-30 motor oil are 4-5 cSt base oil plus a touch of 6 cSt base oil plus Vi improver plus add pack.

How does the small amount of 6 cSt base oil make a major difference in the flow characteristics of a 5W-30 motor oil vs. a 5W-20 oil at normal engine rpm's of 2K to 4K?

To answer the question, it doesn't!

Remember, we are talking passenger car motor oils and not the high load, extreme high rpm's of a race car/race oil application!
 
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