Less oil consumption with THINNER oil?

[moribundman]

Someone I trust when it comes to motors says his Audi 2.8 V6 motor needs about 1 quart of Red Line 30WT every 3k miles, but consumption will increase to 1 quart every 400 miles with Red Line 40WT. He says the thicker the oil, the higher the oil consumption. With lower viscosity oils he also sees more oil in the PCV breather hoses. Keep in mind he uses single weight racing oils.

Usually, it's accepted that thicker oils will tend to reduce oil consumption, so I wonder what's up with this topsy-turvy result? Any thoughts, ideas, theories...?

PS: Does anyone know the HT/HS of RL 30WT and RL 40WT?

 

[DSteven]

It just doesn't seem to follow the norm does it? I had a 96 Dodge Stratus ES (2.5L Mitsubishi built 24V V6) that used about 1qt/2500miles with 10W-30, yet when switched to 5W-30 used hardly a drop. It was always that way, no matter what brand of oil. I called in on an auto enthusiasts radio talk show to ask the question. The hosts insisted I was mixed up, and the consumption would have only been with the 5W and not the 10W. I really wasn't mixed up, but just let the subject go to rest to avoid people thinking differently. Why does this happen?

 

[moribundman]

Maybe the 5W-30 you used was a much better quality oil than the 10W-30?

 

[supraturbo]

My old 98 V6 4 runner was the same way, never burned a drop using mobil 1 5w-30, but burned a qt. of mobil 1 10w-30 in 5k miles, changed back to 5w and no more consumption.

 

[haley10]

quote:

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Originally posted by supraturbo:
My old 98 V6 4 runner was the same way, never burned a drop using mobil 1 5w-30, but burned a qt. of mobil 1 10w-30 in 5k miles, changed back to 5w and no more consumption.

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This may sound dumb and I can't remember the facts, but M1 5-30 is actually thicker over some temperature range. True the HT/HS isn't as good and the NOACK volatility is worse. The 10-30 is more shear stable.

If your engine and driving style was pretty easy on oil, I could believe it. You coulda been running in a range to get a better ring seal. The M1 5-30 is not "thinner" at all temps.

Or you could have been caught between re-formulations.

 

[SteveS]

About 12 years ago I did a small fleet comparison of new and old cars on oil consumption between SAE 15W-40 and SAE 5W-30 grades of the same mineral oil based PCMOs. All cars showed no increase or a lower oil consumption on the SAE 5W-30.

One car in particular (a 1500 cc Ford Laser, a model based on the Mazda 323 and using Mazda mechanicals) had an oil consumption of about 800 ml per 1000 km on the SAE 15W-40. After the change to SAE 5W-30, oil consumption dropped below 300 ml per 1000 km.

There were a number of theories I examined to explain why oil consumption could drop with a lower grade multigrade. Two in particular appeared to explain why an SAE 5W-30 might give lower oil consumption than an SAE 15W-40.

First, one should consider oil film thickness passing the rings between the rings and cylinder wall, and here VII induced temporary viscosity decrease comes into play.

Under shear conditions, the viscosity index improver (VII) strings out and viscosity temproarily drops. This results in a thinner oil film in areas of high shear like the ring/cylinder interface. Therefore less oil would be consumed passed the ring/cylinder interface with the thinner temporary viscosity reduced SAE 5W-30.

However the higher amount of VII needed to make an SAE 5W-30 would mean that the SAE 5W-30 would loose less viscosity at higher temperatures than an SAE 15W-40. A mineral based PCMO which relies on VII for its multigrade characteristics might have something like the following viscosity indices (VI)
SAE 5W-30 155
SAE 10W-30 145
SAE 15W-40 135
SAE 20W-50 125

An oil with a higher VI will thin less at higher temperatures than an oil with a lower VI. At say 150 deg C in the non-shear ring-groove area, an SAE 5W-30 could be in fact thicker than an SAE 15W-40. Therefore there would be less oil loss through the ring-groove pathway, within the ring-groove behind the ring, for an SAE 5W-30 than for an SAE 15W-40, because the SAE 5W-30 is thicker in that high temperature/low shear region.

So over the fraction of an inch width of an oil ring, an SAE 5W-30 would have two advantages over an SAE 15W-40 which would result in reduced oil consumtion.

Now I can't explain the case for reduced oil consumption with the monogrades in the Audi. Maybe the ring/cylinder interface dominates and the thinner SAE 30 oil film between the rings and cylinder contributes more to oil consunmption reduction. However we are talking incredibly low volumes of oil lost passed/through the rings, probably something in the order of less than a hundredth of a thousandth of a ml each stroke.

There is also the likelihood of an increase in oil consumption because of increased volatility with the lighter basestock oils needed to make an SAE 5W-30. But my two theories explained the reason for the reduced oil consumption actually observed between SAE 5W-30 and SAE 15W-40 mineral oil based, VII including, PCMOs.

 

[moribundman]

Thanks Haley10 and SteveS, what you guys are saying makes sense. However, it doesn't explain the difference in consumption between mono-grade oils, especially if they are by the same maker. Maybe there is a Redline rep on this board who could shed light on this issue?

PS: The Audi has low tension rings, so this might have to be considered when searching for answers.

 

[buster]

I've never quite understood oil consumption based on all the responses we get on BITOG. I posted a poll awhile back and the answers were mixed.

I used RL for one interval and my car drank this oil. I can' explain, but SteveS makes some very valid points. RL has very low volatility, as does Amsoil. I think NOAK plays a part, but there are other issues that do as well. It's a weird, quirky problem.

My car used less M1 0w-20, then 0w-30/0w-40 M1. Figure that one out.

 

[haley10]

Yeah we speculate and get off topic sometimes.

With Redline it has to be a total viscosity issue.

I can't see anything other than it being a ring seal issue to consume Redline.

 

[moribundman]

quote:

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I can't see anything other than it being a ring seal issue to consume Redline.

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I would assume that cars with low tension rings are designed for use with thinner oil. However, this is not supported by the suggested oil viscosities in this Audi engines (5W-30 to 10W-60/20W50, with 5W-40 probably being the best compromise).

What's the connection between low tension rings, thin and thick oil, and what if the rings show wear? I think thinner oil tends to increasingly "get away" the more the rings wear, but it seems to be the other way around in case of RL 30WT compared to RL 40WT.

 

[TallPaul]

quote:

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Originally posted by SteveS: However the higher amount of VII needed to make an SAE 5W-30 would mean that the SAE 5W-30 would loose less viscosity at higher temperatures than an SAE 15W-40.

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Fascinating. Never looked at it that way but it seems to explain why the 15w40 I just installed several days ago is not holding oil pressure as well as the 10w40 I was running. The 15w40

1) is dropping from cold pressure (i.e., warming up/thinning) more quickly than the 10w40 did,

2) is more easly dropping below the 44 psi the 10w40 normally stabilized at, and

3) is running 2 to 4 psi lower at idle, in spite of the thicker base oil.

Your theory explains all of this very well.

Also, James Fitch's book "How to Select a Motor Oil or Filter for Your Car or Truck" states (p15): "Do not use a monograde if ... It is possible that [your] engine might overheat."

Always wondered why, but maybe because the VIIs help support the viscosity under high heat.

[ May 24, 2004, 07:32 PM: Message edited by: TallPaul ]

 

[XS650]

quote:

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Originally posted by SteveS:

However the higher amount of VII needed to make an SAE 5W-30 would mean that the SAE 5W-30 would loose less viscosity at higher temperatures than an SAE 15W-40. A mineral based PCMO which relies on VII for its multigrade characteristics might have something like the following viscosity indices (VI)
SAE 5W-30 155
SAE 10W-30 145
SAE 15W-40 135
SAE 20W-50 125

An oil with a higher VI will thin less at higher temperatures than an oil with a lower VI. At say 150 deg C in the non-shear ring-groove area, an SAE 5W-30 could be in fact thicker than an SAE 15W-40. Therefore there would be less oil loss through the ring-groove pathway, within the ring-groove behind the ring, for an SAE 5W-30 than for an SAE 15W-40, because the SAE 5W-30 is thicker in that high temperature/low shear region.

So over the fraction of an inch width of an oil ring, an SAE 5W-30 would have two advantages over an SAE 15W-40 which would result in reduced oil consumtion.

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Using your VI numbers and nominal viscosities at 100C for 30 and 40 weight, 15W-40 is still more viscous than 5W-30 at 200C! That's 392F.

[ May 24, 2004, 08:14 PM: Message edited by: XS650 ]

 

[JohnnyG]

Actually, this was the first information put out by GM when the 1990 or so engines were being developed and put out in production. Theory then said that the thinner oil would "drain out" of the oil ring faster, therby actually reducing oil consumption. At that time, they were advocating 5W-30 oil, which, up to THAT time NOBODY used. As far as getting mixed responses from the board to a question asked here....DUH! There is SO much conflicting info being posted. Not just from us "normal" folk, or those that make their living KNOWING this stuff, but by the engineers, the dealerships, the oil manufacturers, all of them (all of US). The sole purpose of this board should be to sort the BS from reality. Unfortunately, the BS gets more press. The above example was probably the first attempt to "explain away" higher oil consumption more prevelent in the "new design" engines. I group a LOT of engines in to this category, not just from one manufacturer either. There seems to be a change in design philosophy that occured around that time. Whether it be from low tension rings, more VII sticking rings, or higher combustion temps, I can't say.

 

[moribundman]

quote:

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Theory then said that the thinner oil would "drain out" of the oil ring faster, therby actually reducing oil consumption.

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Hmm, between up and down-stroke, how much time does the oil have to drain? Doesn't it simply get wiped off?

 

[Geoff]

Some late model Troopers have oil burning problems. From TSBs it appears related to undersized drainback holes in the oil control ring on the piston.

The problem manifests itself more severely if the engine sustains high rpms (over 2800 rpm) for extended periods, such as driving over 70 mph.

Perhaps the thicker oil gets stuck in the oil ring area for the Audi in the same way and gets burned off instead of draining down into the pan.

 

[moribundman]

quote:

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Perhaps the thicker oil gets stuck in the oil ring area for the Audi in the same way and gets burned off instead of draining down into the pan.

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Well, in my experience, my Audi motor needs definitely LESS oil when using thicker multi-weight oil. I have no experience with single-weight oil, though.

 

[JohnBrowning]

Could it also be that the thicker the oils base stock the more it behaves like a Newtonian fluid?

 

[moribundman]

John, do VIIs make oil somewhat visco-elastic?

 

[ekpolk]

quote:

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Originally posted by haley10:
Yeah we speculate and get off topic sometimes.

With Redline it has to be a total viscosity issue.

I can't see anything other than it being a ring seal issue to consume Redline.

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Have you considered flashpoint? I personally suspect that this may have significant bearing on oil consumption, and may explain anomalies along the grade scale. Look at the concurrent thread on Mobil-1 characteristics. Notice how the very most thin oil, the 0w-20, has the highest flashpoint, followed closely by the 0w-40. This is just hypothetical speculation (can I make it any softer...), but I'd think that this would make the 0w-20 much more resistant to evaporative losses (but not other losses, perhaps?) than the thicker M-1 grades that have lower flashpoints. How do the Redline flashpoints map out? Someone smack me without mercy if my theory is without merit. . .

 

[moribundman]

*smacks ekpolk*

I'm looking at the Red Line race oil spec sheet:

The lower the viscosity, the higher the flashpoint. 30WT has flashpoint of 270 degr. C, 40WT has a flashpoint of 275 degr. Both oils show a NOACK evap loss of 6%. C. Compare that to RL 2 WT, which shows a flashpoint of 160 degree and a NOACK evap loss of a whopping 65%!