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#### Chris B.

Oil thins out when it is hot, right? So when you start your engine the oil is cold and thick, right? So shouldn't oil read 30w10 as it thins out when it heats up? In reality it is the other way around kind of . Can someone explain this?

I think it means that, for examle, a 0W-30 oil will have the viscosity of a 0W oil when cold, but at high temps it will not thin out more than a 30W oil would at the same temperature. So yes, oil thins out with increasing temperatures. I hope I got that right.

I see what you're saying.....why is it when the oil is cold, and thicker than when warm, do they give it a lower number?? In Xw-Y (5w-20 for example), The Xw numbers are based on different temps than the Y numbers. So, they are not comperable.... The numbers themselves do not have any quantitative value.......they represent ranges of viscosity.... I guess they could have used letters instead..... Gotta get my Ew-P!!!

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Originally posted by turbochem: I see what you're saying.....why is it when the oil is cold, and thicker than when warm, do they give it a lower number??
Because multigrade oils are a new technology, relatively speaking. Back in the olden days, there was just plain, straight-grade oil which got one number, say 30 weight. A 30 weight oil, of course, is thick when it's cold and thin when it's hot, but whatever temperature you pick, it's thicker than a 20 weight and thinner than a 40 weight. Now, somebody invents what we call 10W-30. When it's hot, it's as thick as our old 30 weight. But if we put it in the freezer, it doesn't thicken up as much as our 30 weight... relative to a straight 30 weight, it's thin when cold. That's why it gets a lower number, 10W-30. I think you're thinking of "30" as a direct measurement of viscosity. It's not; we measure that in cSt or other units. Just think of "30" as an old-technology reference oil that we compare our oil to, not a reading you'd get from an apparatus that measures viscosity. Edit: I didn't mean to suggest that turbochem doesn't understand this. My explanation was really meant for the original poster. [ July 02, 2003, 03:02 AM: Message edited by: mph ]

If say 0w40 is: Viscosity @ 40C = 80 cSt Viscosity @ 100C = 14.5 cSt Does this mean the oil is 5 times thicker @ 40C?

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Originally posted by Ken4: Does this mean the oil is 5 times thicker @ 40C?
Yes (5.5 times as thick by my math).

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Originally posted by Ken4: But according to this: http://www.bobistheoilguy.com/visc.html 80cSt @ 40C is thinner than 14.5cSt @100C. I am confused.
I don't think you're reading the chart correctly.

Multi-vis oils, whether petro or synth, were designed to have much better pumpability in the winter, thus the lower number on the front-end (5w-30, the "w" stands for winter); the additive package is such that it makes dry start-ups less abrasive on your engine. On the flip-side to that, what they mean by "30" is that under heat and stress the oil will perform like a 30 weight not that it truly is a 30; this sounds a lot like the post up above by "MPH". Multi-vis oils require an additive package, some are more extensive than others, however, you should be looking for an oil with a high TBN and minimal additives; you want your engine protected by the base oil and not the additive package, afterall, the additive package is the first to boil off.

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Originally posted by G-Man II: I don't think you're reading the chart correctly.
80cSt @ 40C = 9.4 cSt @ 100C = SAE 20 ???

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Originally posted by Problem Solver: Multi-vis oils, whether petro or synth, were designed to have much better pumpability in the winter, thus the lower number on the front-end (5w-30, the "w" stands for winter); the additive package is such that it makes dry start-ups less abrasive on your engine. On the flip-side to that, what they mean by "30" is that under heat and stress the oil will perform like a 30 weight not that it truly is a 30; this sounds a lot like the post up above by "MPH". Multi-vis oils require an additive package, some are more extensive than others, however, you should be looking for an oil with a high TBN and minimal additives; you want your engine protected by the base oil and not the additive package, afterall, the additive package is the first to boil off.
Actually, all oils (even straight grades) that meet modern API requirements have an "additive package" comprised of detergents, dispersants, and extreme pressured compounds. Multi-vis oils have an additional additive, the VI improver. (However, some synthetics may not have this if the base oil blend itself has a sufficiently high VI to cover the desired vis spread.)

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Originally posted by Ken4:
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Originally posted by G-Man II: I don't think you're reading the chart correctly.
80cSt @ 40C = 9.4 cSt @ 100C = SAE 20 ???

On this chart, yes. BUT, that doesn't mean that every oil that has a vis of 9.4 cSt @ 100°C will have a vis of 80 cSt @ 40°C. That isn't what the chart is trying to show.

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Originally posted by turbochem: I guess they could have used letters instead.....
Exactly. You will never fully grok the meaning of "double-you" until you give up the notion that the left number of the multi-vis rating bears any relationship to the right number. The SAE should have used letters instead to represent the cold rating. It would have short-circuited much confusion. [ July 02, 2003, 11:34 AM: Message edited by: Jay ]

Yes, oil thins out when hot...as does every other liquid. The numbers don't illustrate the actual thickness of the oil. They just signify the relative thickness 1. at extreme cold and 2. at extreme heat. So, a "0" will be thinner than a 5 at extreme cold and a 40 will be thicker than a 30 at extreme heat. So the numbers are indicative of the oil belonging to a certain "group" rather than a measure of the oil's speicific behavior.

Thank you everyone! I think I have it now!

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Exactly. You will never fully grok the meaning of "double-you" until you give up the notion that the left number of the multi-vis rating bears any relationship to the right number.
That's why when I was a kid, and someone tried to explain multi weught oils to me they said something like "Well, when it heats up, it gets thicker." Obviously that 30 being higher than 10 threw their reasoning a bit off....the oil is not thicker at high than at low temps.....

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Originally posted by Dr. T: Yes, oil thins out when hot...as does every other liquid.
Except for gravy!

Look at this chart: http://www.chevron.com/prodserv/NewOronite/library/li_viscosity_motoroil.htm The viscosities followed by a "W" are tested cold. The viscosities without the "W" are tested hot. So...a 5W-30 passes the cold viscosity test to qualify as a 5W and passes the hot viscosity test to qualify as a 30 wt. Anyone else here old enough to remember straight 20 wt. and 20W-20? The straight 20 wt. was not tested when cold. The 20W-20 passed the cold viscosity test for 20W and the hot viscosity test for 20 wt.. That's all there is to it. Ken [ July 02, 2003, 08:49 PM: Message edited by: Ken2 ]

I see your point totally, but fathom this: Make your own compass with a needle and a magnet. Or take a magnet to a compass, if you know North and South on the magnet. The needle points at North, right? Well, North and North REPEL each other! The long and short of it is that what everyone has been calling The North Pole for the past umpteen decades, is really the SOUTH Pole. So, Winnipeg is 1200 miles due SOUTH of Dallas... Confused yet? Interesting world we live in. Oh, and The Black Hills are neither black, nor are they hills. And you though OIL (30w10) was confusing.

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