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post #1 of 10 Old 04-15-2002, 04:12 PM Thread Starter
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oil: what does 5w30 really mean?

I've searched this forum's archives and found a few threads on oil but no real definition of what "5w30" really means.

I don't think it means "a 5 weight oil that becomes a 30 weight oil at high temperature", because that would imply that it actually thickens at high temperature. And wouldn't that mean higher oil pressure at high temperature? But the oil pressure is highest when the engine is cold, dropping as the engine reaches operating temperature. This would seem to imply that the oil is thinning out at temperature.

Perhaps 5w30 means "a 5 weight oil that loses viscosity like a 30w oil". But what does that really mean? If it's 5 weight when cold, what weight is it at 200 degrees?

My understanding is that all oils lose viscosity with temperature. Multivis oils have chain polymers that unwind as the oil heats up, reducing the viscosity loss. But these polymers cannot eliminate or reverse the viscosity loss, they just reduce it. So multivis oils still lose viscosity with temperature, they just don't lose as much as normal oils. That is, a multivis oil has a flatter viscosity vs. temperature curve, but it's still not perfectly flat.

So can anybody explain what "5w30" really means?

Thanks,


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post #2 of 10 Old 04-15-2002, 04:22 PM
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It's a 5W cold, and acts like a 30W when warmed, and gets no thiner than a straight 30W would at some test temperature. (don't know what that hot temp is)


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post #3 of 10 Old 04-15-2002, 04:42 PM Thread Starter
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Quote:
Originally posted by Ralph Greene
It's a 5W cold, and acts like a 30W when warmed, and gets no thiner than a straight 30W would at some test temperature. (don't know what that hot temp is)
Just what does "acts like a 30W with warmed" mean?

Does it mean the 5w oil gets thicker and reaches a 30W viscosity at high temperature? That would seem impossible. You'd see higher oil pressure at operating temperature than when cold, which is not what happens.

Or, does it mean that as the the 5w oil is heated, it reaches the same viscosity that a 30w would would have reached at the same temperature?

The latter case sounds plausible. But if that is true, then the viscosity of the 5W oil, when it is heated, is actually lower than 5w. In that case, what is that viscosity? Somewhere between 0 and 5?

This would mean there is a crossover point where some very high viscosity oil, say 75 weight, when heated, has a viscosity equal to the lower number. Suppose it's 5w75. If it were possible to create such an oil, it would have the same viscosity at all temperatures. It would mean the oil pressure would depend only on engine RPM and not on temperature.

Anybody know at what viscosity the "crossover point" occurs?

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post #4 of 10 Old 04-15-2002, 04:58 PM
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Maybe the oil guy will come on and tell you what happens to 30W at hot temps. That's what you are asking, right? I don't know if the straight 30 wt ever gets (if it stays in formula) that low vis.

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post #5 of 10 Old 04-15-2002, 05:06 PM Thread Starter
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Quote:
Originally posted by Ralph Greene
Maybe the oil guy will come on and tell you what happens to 30W at hot temps. That's what you are asking, right? I don't know if the straight 30 wt ever gets (if it stays in formula) that low vis.
Yup, that's exactly what I'm asking: what viscosity does 30W oil drop to at temperature?

My gut feeling tells me that 30W oil must reach less than 5w viscosity at temperature, because otherwise the 5w30 oil would actually gain viscosity as it heats up, and we know it doesn't do that else we'd see higher oil pressure on a hot engine, than on a cold engine.

So then the next question is, if it's true that 30w actually drops to less than 5w at temperature, then what "cold" viscosity does reach 5w at temperature? That's where my 5w75 example is coming from.

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post #6 of 10 Old 04-15-2002, 05:15 PM
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The first number is the oil's "room temperature" viscosity, for lack of a better term. The second number is the oil's viscosity at average engine operating temperature (what temperature that average is, I do not know). Multigrade oils, such as a 10W30 oil, are formulated to flow rapidly to areas requiring lubrication when the engine is cold but also to maintain enough viscosity to protect the engine at higher temperatures and operating loads. The number before the 'W' (for Winter) is the oil's viscosity when cold. The number after the 'W' is its viscosity at operating temperatures.

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post #8 of 10 Old 04-15-2002, 05:58 PM Thread Starter
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Thanks, that is the best information I've seen so far. But it still doesn't answer the question.

The relevant text:

"SAE 30 @ 100 degrees C: 9.3 cSt - 12.5 cSt.

"SAE 40 @ 100 degrees C: 12.5 cSt - 16.3 cSt.

"SAE 50 @ 100 degrees C: 16.3 cSt - 21.9 cSt.

The remaining question:

What is the viscosity in cSt @ room temperature (20 degrees C) of: SAE 5, SAE 10, SAE 20, SAE 30, SAE 40, SAE 50 ?

Here is some data that I found, various measurements of different oils taken at 40 degrees C:

SAE 5: 58.4, 59.5
SAE 10: 64.0, 70 (@ -30 C),
SAE 20: 165.9

Conclusion: all oils thin out at temperature. A 5w30 oil loses about 80-85% of its viscosity from 40C to 100C.

Thanks for the info!!!

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post #9 of 10 Old 04-15-2002, 07:55 PM
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It is a 5 weight oil that has thickers added. These thickeners make it thin out at a slower rate than if it was a straight 5 weight oil. In turn this means it behaves like a 30 weight oil at temperature. The oil does thin with temperature, it just doesn't thin as much.

These thickeners are long chain polymers and can become less effective with time. The rule of thumb is that the smaller the range from the lower to the higher number the more stable the oil is in keeping the advertised viscosities. Hence a 5W-20 would be keep its characteristics better under high heat and shear than 10W-40.
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post #10 of 10 Old 04-15-2002, 08:10 PM Thread Starter
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Quote:
Originally posted by IslandCobra
It is a 5 weight oil that has thickers added. These thickeners make it thin out at a slower rate than if it was a straight 5 weight oil. In turn this means it behaves like a 30 weight oil at temperature. The oil does thin with temperature, it just doesn't thin as much.
Right. Which means that for any given "cold" weight such as 5 or 10, there is (at least theoretically) some very high viscosity that thins to this viscosity at high temperature, so that oil, when warm, has the same viscosity as this oil, when cold. That is what I was referring to as the "crossover point".

But it turns out that 60 weight oil is never thicker than 26.1 cSt at high temperature, and 0 or 5 weight oil is never thinner than 50 cSt at room temperature, so the "crossover point" for a 0 or 5 weight oil would be higher than 60 weight oil. And the SAE doesn't go higher than 60 weight for engine oil (it does, but only for gear oil).

If the crossover point were reached, then we would have a "perfect" oil that does not thin with temperature. But this does not exist. Even a 0W60 oil (if such an oil is chemically possible) would lose half its viscosity at high temperature, from about 50 cSt to about 26 cSt.

That's what I was looking for: how much do these oils actually thin out from cold to engine operating temperature. Here are the rough numbers:

A 5w30 oil loses approximately 80-85% of its viscosity.
A 10w40 oil loses approximately 75-80% of its viscosity.

That's when new and fresh. They lose more as they get old and start breaking down.

It would be a LOT more logical and intuitive if they just relabeled a 10w40 oil as 65-15. This would represent its ACTUAL viscosity when cold and warm.


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