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Viscosity Breakdown?

Viscosity Breakdown?

Viscosity Breakdown?

Have a case where a Duramax pickup pulls an hour long grade on the Nevada/California boarder frequently. It's turbo'd and as normal for its ilk has a plate heat-exchanger for oil cooling bolted to the side of the engine using radiator water for the thermal dump. Oil is synthetic.

What's happening is the normal rock-steady 50PSI oil pressure steadily declines over the hour long climb down to 30PSI. Speed never changes, water temperature never rises, just the oil pressure declines. Once the summit is reached the oil pressure recovers over several minutes back to exactly 50PSI.

Having discussed this with five oil company reps several say 'this isn't viscosity breakdown because the pressure recovers'. I tend to agree. I think this is more along the lines of the oil pressure regulation actually causing the pressure drop because of either adverse thermal response or from a small (normal) viscosity drop.

Oil reps are suggesting the need to blend synthetic oil with non-synthetic at this point.


Keith Cress
kcress - http://www.flaminsystems.com

RE: Viscosity Breakdown?

Either the oil is simply getting too hot due to high engine load or it is becoming aerated. The water temperature is regulated by the thermostat so it will not necessarily rise as the oil temp rises. The oil returning from the turbo will be much hotter than the average oil temp. The oil may be getting aerated due to the high engine RPM, and possibly because your oil level is too high. Oil expansion due to high temperature will raise the oil level.

I recently replaced the oil and EGR coolers on my 6.0L Ford diesel with an aftermarket kit from Bullet Proof Diesel. The OEM oil cooler had well known issues with the water passages in the oil cooler plugging due to deposits from the coolant corrosion inhibitors. This restricted coolant flow to the EGR cooler, which would over-heat and crack.

RE: Viscosity Breakdown?

I think adding an oil temp sensor before the cooler, or multiple sensors in other places as well, and then recording data would go a long way towards figuring out what is going on.

RE: Viscosity Breakdown?

Well, it can't hurt to add an oil-to-air cooler.

I'd call that grade severe service.

No way would I mix syn and dino oil, though I am using syn-blend in 5.4l 3V gas engine.

Mike Halloran
Pembroke Pines, FL, USA

RE: Viscosity Breakdown?

i see no need to blend the oil used with a non-synthetic type. it would only reduce the performance characteristics of the actual lubricant.

some small reduction in the oil pressure as measured by the pressure gauge would be quite normal under the circumstances described. the very speedy recovery as described is not something that would be expected.

there are a few aspects here to consider. firstly the oil pressure gauge is not a precision instrument - a certain drop in viscosity may actually lead to a situation where the pressure gauge is no longer capable of measuring with its "normal accuracy". secondly, the oilpressure is no clearcut indication whether the engine is lubricated well - as it obviously is. what is measured is the pressure needed to force the oil through the oil lines to the places where lubrication is needed - bearings, valve actuators etc. the rest of the engine most times is lubricated by the oilmist that is put out by bearings etc and used pistons, rings and liners, the actual pressure in the oil where lubrication is needed can be much higher - a pressure of several hundreds of bars in the bearings is not unusual, and in the contact between camshaft and tappets can be still a lot higher then that. thus, the oil pressure as measured is no indication of what actually goes on in the engine, the pressure measurement device merely is a cheap alternative for what you would like to have: a flowmeter that indicates whether oil is flowing through the lubrication lines....

to find out what is happening, there are a few things you can do. if you take the oil pressure gauge out you might be able to test it and maybe there is a viscosity where the oil gauge no longer is capable of giving a good indication of the pressure.

another point to consider is whether oil pressure regulating valves in the lubrication system act strange. if the oil gets thinner, more oil will be forced through the lines and that might lead some types of pressure regulating valves to act erroneously. one regulating valve may be in the oil lines just after the oil pump, another one might be incorporated in the oil filter assembly and maybe even somewhere else within the lubrication system.

the rapid reversing is indeed strange. however, the oil temperature will be raised during the long climb gradually - when you go down again, the engine load will go down considerably and so will the load on bearings. that will lead to a much lower heat input into the oil passing through the bearings and thus that might explain some of the rapid return of the oil pressure. you might argue that the oil temperature within the ringbelt zone is much higher then within the bearings. that is true. however, that applies only to a limited amount of oil. the bulk of the oil heating up occurs in the bearings under heavy load and in the contact between cams and valve actuators. the varying oil temperatures coming out of the bearings will not be reflected in the water temperature gauge, because the water temperature is regulated by thermostats, fans etc. only when that systems capability is stressed beyond it's maximum you will actually see a rise in temperature.

RE: Viscosity Breakdown?

Thanks for the feedback all.

Aerated. I'm not thinking that's the case because the guy is a stickler for details like only filling it to the 'center' of the dipstick full line. And, he religiously replaces filter and oil on schedule. We use his cast out oil for our oil changes since it literally looks exactly like it did out of the original container. And there is an hour in front of that grade and 3-1/2 hours on mostly level road after the grade with no change in speed. If aeration was happening I'd think it would dog the whole route.

I think a strap-on oil temp sensor is a great place to start!

I've suggested the addition of an auxiliary oil-air cooler and it's being considered.

I'll get them to hold up on the oil 'cocktail' until the temp info is in.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Viscosity Breakdown?

I'd bet a quid on temperature too. Nothing chemical is reversible. Oil temperature is the unknown degree of freedom here.


RE: Viscosity Breakdown?

Does the oil cooler have a thermostat in it? On a number of racing applications, I've seen stacked plate HX's have enough restriction in them to cause similar pressure drops. Once the oil cools down and t-stat closed bypassing the HX, pressure would climb again.

RE: Viscosity Breakdown?

How long is the period over which oil pressure drops from 50 to 30 psi?

Does it happen gradually over the whole length of the grade, or very quickly at some point?

A thermostat doesn't open slowly over a long time- once the activation temperature is reached it will open quickly or close quickly.

RE: Viscosity Breakdown?

Remember we are interested in engine rpm not vehicle speed. It is quite likely that you are in a lower gear going up a steep grade.

RE: Viscosity Breakdown?

Got some clarifications:

I wrote 50PSI it is actually 60PSI so replace all instances of 50 with 60.

When the engine is cold the pressure is 80+PSI as it comes to operating temperature it drops to fixed steady 60PSI unless the hill-pull occurs.

The pressure drops from 60PSI down to 30PSI over the first 15 minutes of the hill then remains at 30 for the next hour of the rest of the climb.

The pressure recovers over a period of 5 minutes once the summit is reached. It's not sudden but gradual.

Lucas Oil, Shell Oil, and Chevron engineers all suggest using Synth/Dino blend.

Chevron Engineers feel the oil is not having "sheer damage" occurring as they insist the oil pressure would not recover ever.

Chevron has instructed the use of their XLE 15-40.
Chevron XLE 15-40

Currently investigating a place to put a temp sensor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Viscosity Breakdown?

How steep is the grade? Perhaps it is steep enough for the oil in the pan to touch the back of the crankshaft, resulting in foaming?

je suis charlie

RE: Viscosity Breakdown?

Gradual drop seems to rule out thermostat. Sounds like oil temperature increase/decrease. What year is it? Older Duramax owners manuals recommended straight dyno oil, at some point they changed to ACDelco dexos1 Synthetic Blend. So going from straight dyno oil to a blend might help. Personally, I'd use full synthetic group 4 oil, not that group 3 BS.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Viscosity Breakdown?

Quote (itsmoked)

Currently investigating a place to put a temp sensor.

While you're looking, see if there is room to get in and out temps for both oil and water going through the exchanger.

I'd like to see a plumbing schematic to understand where everything is relative to pumps and thermostats.

The round numbers and steady pressures make me think internal relief valves.

RE: Viscosity Breakdown?

I suspect it's just a temperature effect. Where going up the hill where the load is higher means higher turbocharger temperatures and higher oil temperatures exiting the turbocharger than when more or less coasting down the back side. It may even be possible that the rejection of more heat through the coolant radiator and exhaust piping is raising the temperature of air passing along the sump enough to reduce heat rejection through that component.


RE: Viscosity Breakdown?

Agree with dgallup. Full syn group 4 is going to have the best viscosity index - i.e. least change in viscosity due to temperature changes. Dino is usually the worst, not to mention it is much more prone to oxidation. Just part of the reason I run group 4 in my personal vehicles which see -10°F starts in winter. ISZ

RE: Viscosity Breakdown?

What kind of oil rep would suggest blending oil? I would fire them. Have your oil tested for fuel dilution. Try a full synthetic with a better viscosity index. A full hour pull is some pretty severe duty and a full synthetic oil may save money in wear and tear in the long run.

RE: Viscosity Breakdown?

Ruling out fuel dilution is task 1.
I like the sound of going to group 4 also.
You may need to go up slightly in visc. but you need to be careful.
Your temp data should give you guidance on that.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Viscosity Breakdown?

The mineral/synthetic blends are not something new, I used to use syn blend some years back.
Here ya go.

So out of all these words and numbers I see here in this post there is no mention of what viscosity the oil used is.
With the heat soak, something could be leaking as well.

RE: Viscosity Breakdown?

I didn't mean synthetic blends, I meant buying a quart of conventional and blending it with a quart of synthetic. Oil formulations have been changing a lot lately. In the past 3 years there have been 4 different formulations of Chevron Delo 400. It can't be ideal to have an anti-wear package that is half zinc/phosphate based (Delo 400 MG) and one that is half boron based (Delo 400 LE).

RE: Viscosity Breakdown?

If you are going to blend a conventional and a synthetic to make your own semi-synthetic, you'd better make sure they use the same additive package so there's no interfering additives.

Like TugboatEng said, when it comes to lubricant additives, half of one type and half of another type isn't likely to result in good performance from either half. Specific levels of specific additives make an oil perform the way it is supposed to.

The only way I could see the reps suggesting that you mix them is if they know the two types they sell use the exact same additive system so it would really only be a base oil change.

Andrew H.

RE: Viscosity Breakdown?


Well, Chevron XLE 15-40 the synthetic/dinosaur blend was finally acquired. It was a battle as it wasn't apparently for sale yet. It had to be ordered by the western US distributor.

This weekend the oil was changed to the XLE and a run over the Sierra Nevadas on the same exact route, driving the same as always (cruise control 68MPH) up the looooooOOng slope and down the other side.

The results were interesting.

His around town/freeway oil pressure has dropped 5psi from what it was with the prior oil 60 down to 55.
His transmission temperature went up from 175F to engine temp 195F.
His oil pressure on the grade dropped only to 40-45PSI instead of 30-35PSI. Recovering on the downhill just like with the pure dino.
His water temp has stayed rock steady at 195F in all cases.
His radiator fan ran on the grade when it hasn't in the past. (This cost him 1/2MPG.)

It appears this new oil transfers heat better though I'm not clear on how or if that might help it maintain more pressure.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Viscosity Breakdown?

Quote (itsmoked)

It appears this new oil transfers heat better though I'm not clear on how or if that might help it maintain more pressure.

If the oil is shedding more heat, less vicosity change is a possibility, which would maintain higher pressures.

It's also possible that this particular oil just handles the service temperatures better.. temperature/viscosity plots for oils, especially blends, are sometimes very strange, and are MUCh more complicated than the ratings on the bottles make them appear to be.

RE: Viscosity Breakdown?

Oils have a VI, viscosity index which is the decrease in viscosity as temp increases.
But it isn't that simple, I have sen cases where moving to thinner oil actually lowered temps because of less shear in the oil and better circulation.
In your case the new oil may be maintaining viscosity better, but it may flow better to start with which improves heat transfer, which also keeps the oil a little cooler. The combination would be less thinning under load.

If in the past the fan wasn't running it sort of concerns me about the sizing of the cooler and placement of the sensors. It sounds like the system is working properly now.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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