Ok so I know that the 10 in 10w30 is the cold oil viscosity. The w is for winter, so you can use it in winter. The 30 is the viscosity at 210 F. My question is is a higher weight number thinner or thicker oil at operating temp? Would a 10w40 be thicker at operating temp or 10w30? Just curious. I never delved into oil viscosity before.
Normally, an oil will have a predictable viscosity curve based on temperature. All it means when you have a multigrade oil, is that when the oil is cold, its viscosity profile fits that of a 5 or 10 grade oil, which is relatively thin given the temperature. However when it warms up to operating temperature, its grades profile gets higher, meaning that the oil is relatively thick for its temperature. Its still a lot thinner at temperature then it is cold, but it is thicker then it would be if it were a single grade oil.
So, 5W30 Oil and 10W30 oil has the same viscosity at operating temperature. and... 5W30 and 5W20 have the same viscosity when cold.
My 3.4 DOHC tends to like thicker oils, I found I needed to up the cold grade from 5 to 10 to get it to not rattle on startup. And I need at minimum 30 grade at temperature to get the lifters to quiet down. Sometimes thats just how you have to do it.
The w has nothing to do with winter. It means 'weight.'
"Weight" is even less correct than "winter." The 'W' simply indicates a specific set of SAE test conditions, which includes low temperature cranking and pumping as criteria. If you speak only of a "30 weight" oil, it doesn't tell you anything about viscosity unless you also specify a temperature and test method.
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5w30 means it is an oil that is 5 weight, and has the viscosity of a 30w oil.
This is just totally false! From SAE J300:
"Two series of viscosity grades are defined in Table (1): (a) those containing the letter W and (b) those without. Single viscosity grade oils with the letter W are defined by maximum low temperature cranking and pumping viscosities and a minimum kinematic viscosity at 100C. Single grade oils without the letter W are based on a set of minimum and maximum kinematic viscosities at 100C and a minimum high shear rate viscosity at 150C. ... Multi-grade grade oils are defined by both of these criteria...."
KurtAKX has it pretty much right (and used a lot fewer words ). The term "weight" is just an oversimplified indicator of relative viscosity at a particular test temperature ... the higher the number the higher the viscosity of the oil (i.e. higher => thicker). Per the SAE spec, all 30 weight (without the W) oils will have a kinematic viscosity of ~10 cSt at a test temperature of 100 C (212 F). Thus 10W30, 5W30, and 0W30 oils will all have nearly identical viscosities at an engine operating temperature of 100 C. Unfortunately, the same logic does not apply to the 'W' test condition; not all 5W oils will have the same viscosity at 0 C (32 F). A 5W40 oil will be considerably thicker (in terms of actual viscosity) than a 5W30 oil at a typical startup temperature of 24 C (75 F). A 5W30 will still be thinner than a 10W30 at cold startup temperatures, though.
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Originally posted by Curlrup:
Would a 10w40 be thicker at operating temp or 10w30?
The 10W40 will be "thicker." Here are the actual viscosity ranges (in centiStokes) at 100 C (212 F) specified for 20 through 60 weight oil per SAE J300:
Thus a 10W40 oil will be about 30% "thicker" than a 10W30 oil at 212 F.
On the other hand, for completeness, here are the approximate relative viscosities at 24 C (75 F) startup for three multiweight oils, again per SAE J300:
Originally posted by Curlrup: My question is is a higher weight number thinner or thicker oil at operating temp? Would a 10w40 be thicker at operating temp or 10w30? Just curious.
More viscosity (i.e. bigger number) means thicker oil. Generally speaking, hotter climates would require higher viscosity oil, and vice versa for cold climates. So, for example, you might use 10W30 in the winter, and 20W40 in the summer.
Also, keep in mind that friction modifiers are used to increase viscosity at high temperatures. So generally speaking, the bigger the difference between the first and second numbers, the more friction modifiers are used. And since your engine only holds a certain amount of oil, more friction modifiers means less actual oil in your engine. Because of this, I personally prefer to have the two numbers closer together. For example, I would pick 10w30 over 5w30, because the 10w30 would have less friction modifiers. I would also pick 20w40 over 20w50, for the same reason.
Also, I think I should mention ZDDP. It's a Zinc additive that helps lubricate the cam & lifters in our "old school" flat-tappet valvetrains. Most modern oils do not have it (or have very little). I can't remember offhand all the ones that have it. But I use the Shell Rotella, because it's commonly available and relatively inexpensive.
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01:16 PM
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
More viscosity (i.e. bigger number) means thicker oil.
Yes.
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Generally speaking, hotter climates would require higher viscosity oil, and vice versa for cold climates. So, for example, you might use 10W30 in the winter, and 20W40 in the summer.
I disagree. The oil temperature in a fully warmed up water-cooled engine will be dominated by the coolant temperature, which the thermostat will keep nearly the same summer or winter. Climate has even less of an effect on engine operating temperature than the seasons. (This is less true for air-cooled engines, where ambient air temperature has a significant effect on engine cooling.)
On the other hand, cold start is determined by ambient temperature, which is strongly dependent upon climate and season. But there is still little or no reason to use different oil viscosities in different seasons. Even a 0W oil is thicker than optimum for cold-weather startup. At cold startup a lower 'W' number (within the range specified by the engine manufacturer) is almost always better, at any ambient temperature. Remember that about 90% of the wear accumulated in a street-driven engine occurs during the first minute or so after startup.
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Also, keep in mind that friction modifiers are used to increase viscosity at high temperatures. So generally speaking, the bigger the difference between the first and second numbers, the more friction modifiers are used.
Friction modifiers have little or no effect on viscosity. Did you perhaps mean "Viscosity Modifiers" (a.k.a. Viscosity Index Improvers)? Viscosity modifiers are necessary to obtain multiviscosity performance in conventional (refined petroleum) oils, but synthetic oils have a much higher viscosity index to start with and generally need no VI improvers at all.
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... I personally prefer to have the two numbers closer together. For example, I would pick 10w30 over 5w30, because the 10w30 would have less friction modifiers. I would also pick 20w40 over 20w50, for the same reason.
There was a time in the late 1970s when this was a valid concern with some engines (particularly GM, which recommended 10W30 but prohibited 10W40), but even then it only applied to conventional motor oils, not synthetics. But by logical extension of your argument, the "best" oil would be a 30W30 (or 40W40 or 50W50), which is clearly not the case. The "best" oil viscosity range to choose is the one recommended by the engine manufacturer.
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Also, I think I should mention ZDDP. It's a Zinc additive that helps lubricate the cam & lifters in our "old school" flat-tappet valvetrains. Most modern oils do not have it (or have very little).
This is a legitimate concern, but primarily when breaking in a new cam and lifters or if you have modified your engine from stock ... e.g. high-rate valve springs and/or high-ratio rocker arms, both of which increase the load seen by the cam and lifters. It is much less an issue with a mostly-stock engine after the break-in period. I also think the new API "SM" oil standard attempts to address this wear issue somewhat. This is also a case where an oil with a lower cold viscosity may be desirable, because it will get oil to the cam and lifters faster than a thick oil after startup.
[This message has been edited by Marvin McInnis (edited 10-07-2010).]
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01:58 PM
Blacktree Member
Posts: 20770 From: Central Florida Registered: Dec 2001
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
). The term "weight" is just an oversimplified indicator of relative viscosity at a particular test temperature ... the higher the number the higher the viscosity of the oil (i.e. higher => thicker). Per the SAE spec, all 30 weight (without the W) oils will have a kinematic viscosity of ~10 cSt at a test temperature of 100 C (212 F). Thus 10W30, 5W30, and 0W30 oils will all have nearly identical viscosities at an engine operating temperature of 100 C. Unfortunately, the same logic does not apply to the 'W' test condition; not all 5W oils will have the same viscosity at 0 C (32 F). A 5W40 oil will be considerably thicker (in terms of actual viscosity) than a 5W30 oil at a typical startup temperature of 24 C (75 F). A 5W30 will still be thinner than a 10W30 at cold startup temperatures, though.
