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Here is a good link to begin with when trying to understand oil viscosity, especially if your wondering why one engine calls for 0W-whatever and another calls for 5W-whatever. Seems the only difference between the bottom 2 numbers is the cold pour temp. Engine oil, like most other car products like additives are wide open to a wide and varying range of opinions on what is right. I've read various different opinions on this, so much so that it can make your head spin. For me, the most important thing is....change your oil frequently. 2,500 to 3K mi. for regular oil & 3,500 to 5K for synthetics, or every 3 to 4 months if you do not drive often enough to hit those miles during that length of time. Hope this link helps.

Car maintenance bibles: Oil Viscosity.
 

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Just keep in mind, oil gets incredibly complex, you can't sum it up into simple concepts like that. It is correct, if you only examine the oil at the high and low temps they measure those ratings at, what about the whole range of temps inbetween those two measurements or temps higher and lower?

I've read in more than one spot, there are far more difference between a 0W-30 and 5W-30 than the cold pour point.

Every oil starts as a base stock oil with certain properties, that they modify it with additives to give new different properties. So in simple terms, you could say the only difference between the 2 oils is just the cold pour point, because thats what the numbers mean. BUT, on the other hand, if you look at all the data and the viscosities for various temps along the whole range, you'd see it is different. It NOT linear either, with varying curves for the changes.

You likely would see at 50°C, the 5W-30 has a higher viscosity than 0W-30. Plot a line for 0 to 30 and another line for 5 to 30, you'll notice the line from 5 to 30 is higher for the same temp all the way till they meet at 30. In reality its a curve and NOT a smooth curve the but the same basic principle applies. And since 5W-30 is a shorter spread between the two, it often needs less additives to modify its properties and that can be advantages, because its usually the additives that burn and leave the deposits and sludge in the engine. Then, what about if the oil goes above the 100°C temp that they measure the higher viscosity number? NOT always, but often the oil will less additives and better base stock will go to higher temperatures and hold its viscosity better and NOT drop off in viscosity as fast at the higher temps.

I'm only scratching the surface, and I am hardly an expert, in fact I could be wrong on some of this stuff. The only thing I can be sure of, in oversimplified terms what you say is true, in reality its more than that, you'll find more difference between 0W-whatever and 5W-whatever, other than the pour point.

BUT, it seems to be a proportional difference, even though there are more difference then you say, they are really slight for a slight change in one of the viscosity numbers.

So basically, I'm splitting hairs, BUT, a lot of oil choices come down to splitting hairs. Thats why I stick with the manufacturers recommendation and don't try to out think it, saying well the only difference is the low temp viscosity, there are some other difference and slightly higher viscosities at other temps that might be a better benefit.

For example, If you have a vehicle that recommends 20W-40 and you decide to use 0W-40, because the only difference between the two is the cold pour point, whoa, 20pts is a pretty big difference, and I think you'll find you'd get very different oil pressure (which means different volume and flow) all the way up to around 100°C and the 0W-40 might drop off in viscosity much faster if your engine runs hot and the oil gets up to much higher temps than 100°C.
 

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Thanks Mongo for "weighing" in. I read your post, then went back and re-read mine and I noticed that to someone else it probably read like an oversimplification, for that I am truly sorry, wasn't the point I was really trying to make. To oversimplify my original point in my own words, not technical ones...I'll use my own words..."To me the difference between 0W-30 and 5W-20 is so minor that in a typical temperate climate its almost like splitting hairs...I do stress what I think is the most important part of any oil viscosity choice would actually have nothing to do with viscosity at all but instead the regular changing of oil being way more important than a deviation from one number to the very next. I remember back in the day when we used to run one variable viscosity oil in the summer and a different one in the winter because the lower number stayed more fluid in the brutally cold winter months here in Ohio allowing your cold engine to turn over and start faster. Times have changed, gotten way, way, more complex, but I am pretty sure we are still manufacturing engines outta the same metals on the inside so I wonder how much (other than cost and product choice has really changed?) Regular oil changes are the key."
 

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..."To me the difference between 0W-30 and 5W-20 is so minor that in a typical temperate climate its almost like splitting hairs...
That makes sense, I'm definitely NOT enough of an expert to say if that statement is right, my guess you would see a difference, BUT I really don't know if its enought to really effect the performance or longevity of the motor.

My guess, also, although some difference in oil pressure and flow, its probably still within the acceptable range for the motor for the conditions most of us drive.

Like you're implying, I'd agree, what would make more of difference in protecting the motor is the quality of the oil/filter and how often you change it.
 
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