why worry about the low #?

[JHZR2]

I have yet another question regarding choosing oil viscosities. Say a vehicle calls for 10w-30 oil. OK, thats all well and good, there are a lot of good types around. But, say you know a little about oil, and want to squeeze out a few extra MPG. Would the change to 5w-30, or even 0w-30, assuming that you get a high quality oil (mobil 1) with no or high quality VIIs, make much if any difference?
I cannot imagine that the protection provided by a 0w-30 oil in an engine designed for 10w-30 could possibly be much less.
Is there a guideline for allowable spread, assuming that a high quality oil is chosen, for what is reccomended and what is used? Say my car uses a 15w-40 oil for the most part. WHat is the effect if a 0w-40 oil was used instead, all the time? Why do people talk about an oil being too light for their climate? A 5w-30 oil may be really thin at startup in 110 degree weather, but whats the problem with that? And, since engine temperature is controlled, and at steady state is more or less the same, then all that really matters is to get the top number correct for your engine design, and not have the low number be higher than the MFR's reccomended (but lower is OK).

Is my reasoning correct here?

My second, related question is regarding oil weight specs, and engine design. What is the important thing, the ability for oil to flow through the engine, or the ability to have a certain viscosity? There are all sorts of numbers provided on the spec sheets, but I am not very knowledgable as to what are really important to the engine design and oil specification. At low temperatures, is it more important to have something that pumps well, or is it more important to have something with a certain viscosity (both things are related, but synth oils, for example, pump well in the cold, but have essentially the same viscosity as a dino oil). At high temperatures, I guess most specs are the same, or unimportant for the most part (assuming your engine is healthy and not too hot), except shear stability. so then the viscosity chosen by the MFR. is based upon bearing clearances almost entirely, correct?

I am sure these are relatively simple questions, but Im trying to learn as much as possible for my own ood, and thank you for your help... this site is great!

JMH

 

[G-MAN]

quote:

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Originally posted by Dr. T:
Remember, all oils are one base viscosity with addative to allow for the spread. These addative are NOT lubricative in nature and break down leaving just the base oil grade. Narrower ranges have less of these addatives necessary for "the range". Hence, both numbers contribute to this range.

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This isn't quite correct. The VI of the base oil (or base oil blend) can be high enough in some cases that no VI improver is needed to achieve the desired spread of the finished motor oil. Red Line seems to indicate that is the case with their oil, and MoleKule has said that ExxonMobil has the ability to combine various synthetic base stocks to obtain just about any viscosity spread without the need for VI improvers.

 

[MolaKule]

"And, since engine temperature is controlled, and at steady state is more or less the same, then all that really matters is to get the top number correct for your engine design, and not have the low number be higher than the MFR's reccomended (but lower is OK)."

Mostly correct. You also have to consider those short-lived accelerations or bursts of power which produce intense combustion temps and heating for short periods of time. However, the coolant and head temperatures lag in catching up with these temperature spikes, so the oil is called upon not only to lubricate, but to cool these components. Steady state temps are only reached tens of seconds later. Without a good oil and thick oil film, hydrodynamic lubrication is breached and boundary lubrication is required; mostly through the use of FM's and anti-wears additives, such as esters, ZDDP, and Moly.

So oil has two reasons for being: lubrication and cooling.

 

[Dr. T]

I was referring to oils such as 5-50...don't tell me they don't have any VI's??

 

[G-MAN]

quote:

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Originally posted by Dr. T:
I was referring to oils such as 5-50...don't tell me they don't have any VI's??

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I think it is possible to make a 5w50 oil without VI improvers simply by blending the right synthetic base stocks. Whether any manufacturers actually do that, I don't know.

 

[nick778]

So, other than for fuel efficiency, oil stability and low temp flow reasons, what determines the upper limit or high temp oil weight (i.e., xx-30/40/50) OEMs spec for their oil in their engines? I presume it has something to do with clearances and that 'too thick' a viscosity at operating temp might actually increase temperature and decrease performance due to increased friction(?).

 

[Dr. T]

I think it has more to do with philosophy. ie. U.S. mentality...use 5-30 and change every 3k...Euro...use 20-50 and change every 8-10k. Even the same cars with the same engines have different oil recommendations on 2 different continents....the engine is certainly the same...or different year...different grade recommendation...eg. 5-30 to 5-20 in Fords...

 

[G-MAN]

quote:

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Originally posted by Dr. T:
I think it has more to do with philosophy. ie. U.S. mentality...use 5-30 and change every 3k...Euro...use 20-50 and change every 8-10k. Even the same cars with the same engines have different oil recommendations on 2 different continents....the engine is certainly the same...or different year...different grade recommendation...eg. 5-30 to 5-20 in Fords...

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I don't know of any European makers who recommend 20w50. 0w30 and 0w40 are the predominant grades. 5w30 is primarily what comes in Mercedes vehicles, with 0w40 Mobil 1 coming in the AMG models. Mobil 1 0w40 comes in Porsches and Aston Martins. Shell Helix Ultra 5w40 comes in all Ferraris. VW and Audi come with 0w30. The latest GM long life OEM European oil is a 0w30.

 

[Dr. T]

I see your reasoning...and agree that the higher number is more important because it signifies the oil's viscosity at temp.. This is important because it means it will provide an adequate film thickness at operating temperature.

However, you need to remember that overall, eg. 5-50 when compared to a 15-50 will be 1. thinner to begin with and 2. more prone to thinning out because of it's wider spread will have it more prone to viscosity breakdown with mileage/age.

Remember, all oils are one base viscosity with addative to allow for the spread. These addative are NOT lubricative in nature and break down leaving just the base oil grade. Narrower ranges have less of these addatives necessary for "the range". Hence, both numbers contribute to this range.

Secondly, I think the "flow" thing is overrated. An oil doesn't need to "flow" through your engine like a waterfall. It it "pumped" through the engine in order to maintain a lubricative film over moving metal parts. All hot oils will "flow"...therefore, film thickness is utmost in providing protection. What flow will a thin oil and/or particulates have when it's seared on piston rings? Or cooked on engine parts? Zero. Hence, power robbing sludge begins a downward cascade...

 

[Dr. T]

Well, that's what I'm saying...who does Castrol make the 10-60 for? Only for the BMW M3/M5? Why bother when a 0-30 is so great? Mobil 1 has both a 5-40, 5-50 in addition to the 15-50 in Europe. A friend of mine in Germany laughs at the BMW 5-30 recommendation in the U.S.. Told me nobody uses a 5-30 here.

 

[vvk]

quote:

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Originally posted by Dr. T:
Well, that's what I'm saying...who does Castrol make the 10-60 for? Only for the BMW M3/M5? Why bother when a 0-30 is so great? Mobil 1 has both a 5-40, 5-50 in addition to the 15-50 in Europe. A friend of mine in Germany laughs at the BMW 5-30 recommendation in the U.S.. Told me nobody uses a 5-30 here.

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5W-30 is not sold, that's why nobody uses it. In the US if you go to Wal-Mart you will not find 0W-40 or 5W-40. So almost nobody uses these. In Europe it's reverse: all you can find in stores is 0W-40 and 5W-40. So that's what people use. You will have to look very hard and long to find 5W-30 in Germany.

10W-60 is a "motorsports" grade. People buy it because it's "cool". Wow, it's 60 weight, it's for Dakkar rally, must be good stuff... That's the thinking...

 

[widman]

During engine start-up and shut-down (or any equipment), the hydrodanamic lubrication is weak or nonexistent. You have to build a hydrodynamic cushion (explained very well on this site) to float everything in the varios bearings. if the oil is too thick, it doesn't flow, so you have a poor hydrodynamic cushion. If it is too thin it will flow through the bearing too quickly and not form the cushion.
The faster you build this cushion the less wear you have.

 

[GEMINIS2k]

So why the different weights same engines different weights and both cars with the same motor runs well and in Europe with all there Exotics they have there they use 5w40 while the US is always 10w30 is it because they run over 100mph+ in europe and we run 65mph and would never reach high speeds???

 

[widman]

As far as I know all 5w40's are synthetic. that helps the low temp and high temp, reduces shear, helps high speed driving that is more prevalent there than the US. But the US is also concerned with CAFE, so tend to stay on the lower end of what works.