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You mis-read at BITOG.com....
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Posted by nwas (more from nwas) on Thu, 22 Dec 2011 15:57:45 Share Post by Email
In Reply to: This statement does make sense if you read...., S-Works, Thu, 22 Dec 2011 08:57:59
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You are mixing a couple concepts, but I compliment you on starting to learn. First, there are two oil viscosity measurements:

1. The cold rating, designated by a "W" like in 5W-??
2. The hot rating - the second number i.e. ?W-40.

The two ratings are in no way comparable and use a completely different measurement system. Some people say "I wouldn't run my car on a 0W oil", but that just shows that they don't have a clue :-)

The hot rating is viscosity the oil will have at operating temp (100 deg c). A ?W-30 oil will be the viscosity of a straight 30 weight at 100c. A ?W-40 weight oil will be the weight of a straight 40 weight at 100c. So, the second number DOES matter in terms of how thick your oil is at operating temperature.

The first number is the "Winter" weight. Again, it is in no way comparable to the second number. Different scale. Apples and oranges. If you pour a 0W oil cold and measure its viscosity, you will find that it is actually physically thicker than a 40 weight at operating temp.

In fact, you can't buy an oil that's really thin enough at startup (cold), but 0W is almost there. Estimates are the 80-85% of engine wear occurs at startup because traditional oils don't get flowing fast enough and your engine runs "dry" until oil starts flowing. At startup, "flow" is the key, not "pressure", but pressure will build quickly as the flow begins.

Back to your statement about "it's designed to be a specific thickness at operating temp. IE: 40w is not thicker than 30w at operating temp": what you should say is that " IE: 40w is not NECESSARILLY thicker than 30w at STARTING temp". It's not, it's the lower (Winter) number that designates the viscosity at startup and the upper number is unrelated.

Make sense?

posted by 173.76.11...

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