Would it by worth it for me to switch to synthetic oil with 125k miles on my 5.9? Why or why not?

I like synthetic because of its flat viscosity curve. It doesn't thicken up as much when cold, so the engine gets better startup lubrication. It doesn't thin out as much at higher temps, so the engine keeps its oil pressure a little better.

I also like it because many high performance cars come with it from the factory. I respect the decisions of the factory engineers.

I'm a former lube engineer (Shell Oil) and Navy Fuels and Lube Technical Officer (1 of 6 in the USN).

Firm, unequivocal answer: "It depends"

The two main reasons to change oil are base stock deterioration and depletion of the additive package.

Synthetic base stocks are less susceptible to deterioration because unlike mineral oils they are homogeneous and contain no undesirable compounds. Synthetics were first developed for jet aircraft due to the demands of the extremes in which they operate.

Mineral base stocks can be refined to eliminate many of the undesirable compounds. What is not widely understood is that some "contaminants" allow a more robust additive package to be employed.

In contemporary gaso vehicles, the use of fuel injection and unleaded fuels pretty much eliminates concerns over additive depletion, so the longevity of synthetic base stock is a benefit.

For diesels, not so much, as the oil is usually discarded when the additives are depleted.

While the feature of a (slightly) more robust Viscosity Index is a theoretical benefit, it really isn't in a diesel motorhome, because it isn't a daily driver, and when it does hit the road, it usually for a longer trip.

Bottom line: IMHO, is that synthetics are wasted on diesel coaches. I use Rotella T 15W40 in both my van (6.9 NA diesel) and Barth (6.5L TD). I would use Rotella 30 in the non-turbo van if it were readily available. The Lexus gets Mobil 1, because sometimes it sits for several weeks. When it was a daily driver, I used mineral-based oil.

BTW, bill h, I know you'll be shocked to know there are "promotional considerations" for the high performance car manufacturers to use certain products...an exception in the '60s and '70s was the Gulf Oil Ford GT40s - which used Shell oil because the Gulf products wouldn't hold up...

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Excellent post Rusty. Good information and I hold exactly the same beliefs about the additives. Change oil twice a year in diesel engines in motorhomes is a good idea IMHO. When I had city tractors they used to get fresh oil every 90 days. The highway tractors every 10 to 12 thousand miles or about once a month. There was once a study that showed you could pour fresh oil in an engine that had been run then don't start it just let it sit. The additives were depleted after time just from the toxins, acids and other contaminants in the crankcase due to the process of internal combustion.

I wanted to wait until Rusty posted because I know he's a true expert in this area.

Several things that are bantered about in the automotive community that seem sensible and true that I'll add, 1) some engines should be allowed to break in on standard oil then switched to synthetic if that is your goal. The synthetic is so good at lubrication that the engine won't seat in the friction based parts. 2) synthetic is thinner, which can cause problems with older engines. Leaking or low oil pressure can be experienced when switching to synthetic. 3) In very cold regions, synthetic has better lubrication as it doesn't thicken as much as dino oil in cold.

These are the urban legends anyway, they all seem logical to me.

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Bottom line: IMHO, is that synthetics are wasted on diesel coaches. I use Rotella T 15W40 in both my van (6.9 NA diesel) and Barth (6.5L TD). I would use Rotella 30 in the non-turbo van if it were readily available. The Lexus gets Mobil 1, because sometimes it sits for several weeks. When it was a daily driver, I used mineral-based oil.

While I generally agree with this statement and position, I have found in my 5.9L oil presurres are higher and stay higher for longer periods of time using Kendell Synthetic blend ( I know a lot of banter about "blends"). I have done extensive comparison using Rotella, Mobile, and Delo. good pressure to start after oil change and then about 4-5K miles later, at least 4-6 pounds down at the same RPM and engine temperature. With Kendell, pressure is at least 5 lbs higher to start and will drop 2-4 lbs. After 4-5K miles, Kendell carries higher pressure than the other brands do at oil change start.

Richard if you are talking about synthetic change for the trans, if you don't run in high temp conditions for long periods of time and change regularly, you will probably be OK using dino ATF fluid. In my case, as I mentioned, the oil and filters had NEVER been changed for the first 75k miles and I run hard, fast, heavily loaded, and in the desert heat. After switching to synthetic oil (2 changes) the trans temp is lower under similar condition by 15-20 degrees. After th 3rd change no further reduction in temp.

Switching to synthetic oil in the trans will take at least 3 changes to get most of the old non-synthetic oil diluted and out of the system.

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Originally posted by MWrench:

Switching to synthetic oil in the trans will take at least 3 changes to get most of the old non-synthetic oil diluted and out of the system.

Some of that will depend on whether or not the TQ has a drain plug.

Or rivet.

Since building race engines is one of my businesses I thought I would toss out a couple of ‘factiods’ for this discussion –
The minimum oil pressure that an engine needs for lubrication is about 5psi at the bearings. More than that will not get you better lubrication. Less will destroy your bearings!

Oil is an important coolant. In some systems it cools as much as the ‘antifreeze’. In fact air cooled engines like the old Porsche and VWs are really oil cooled with some help from the air cooling the cylinders.

Synthetics were not recommended for break in oils because they are too ‘slippery’, during the break-in period newly machined surfaces are wearing against each other to from a good seal. In truth most modern close tolerance machining practices the break-in period is minimal. Most builders still use a Dino break-in oil because you want to change it frequently to make sure any contaminants from the machining processes are cleaned out.

One of the big differences between Dino and synthetic oils is the temp at which they start to break down. As a generalization its not a good idea to run Dino oil over 250f. I have seen some synthetics run over 300f without signs of failure. The only significant reason I can think of to run synthetics in a motor home is if you are going to do some heavy towing in a hot climate.

What I tell my race engine customers is I would rather see them changing their oil more often than trying to run longer with synthetic oils. If you oil is getting contaminated from blow-by, carbon or anything else it doesn’t really matter if the oil is Dino or synthetic – it still is getting contaminated and should be changed.

Modern engines run on much thinner oils because the machining tolerances are much better. The main thing that determines what oil viscosity you should run in main and rod bearing clearances (both bearing and side clearances). This is the main item that determines what volume of oil can flow through the engine. The viscosity needs to be matched to clearances to determine flow rate. You want the correct match to ensure that you are getting both pressure for lubrication and flow for cooling. Too high an oil pressure indicates that the flow is being restricted too much and there is not enough flow for cooling.

One last thing – read the labels carefully on oils labeled ‘Synthetic” – some are only partial synthetic, and they don’t tell you how much “partial” is….could be 1% and they still get to use the ‘Partial’ statement!

Interesting to note that the old Italian and English car manufactures figured out this problem 50 years ago. Those old cars leaked out oil faster than it could deteriorate. If you had to add a quart every week or two it was a similar to a continuous oil change! (I’m kidding….sort of! It was not designed that way, but that’s the way it worked!)

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The minimum oil pressure that an engine needs for lubrication is about 5psi at the bearings. More than that will not get you better lubrication. Less will destroy your bearings!

Oddly enough, the higher the RPM, the LESS pressure is needed.

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Synthetics were not recommended for break in oils because they are too ‘slippery’, during the break-in period newly machined surfaces are wearing against each other to from a good seal.

There is almost immeasurable difference between the "lubricity" (this term is thrown about liberally in the lube community, yet has no accepted definition) of synthetic and mineral oil. I suspect this is a myth carried over from the recip aviation community, where non-detergent oil is recommended after top overhaul or replacement of a jug to allow seating of the rings.

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The minimum oil pressure that an engine needs for lubrication is about 5psi at the bearings. More than that will not get you better lubrication. Less will destroy your bearings!

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Oddly enough, the higher the RPM, the LESS pressure is needed.

I do not agree with either of these statements as applied to vehicle engines. Most all of the performance engine builders agree that a minimum of 10 PSI/1000 RPM is need to insure lubrication and prevent engine failure. While pressure isn't the real factor, it is needed to insure that the film of oil between various rotating metal parts is thick enough to provide a barrier and prevent metal to metal contact under ALL circumstances. Turbo charged engines or very high compression engines present more of a lubrication challenge as well.Any brief loss of pressure/oil volume or air in the system will and does cause bearing failure almost immediately.

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Originally posted by chrisW:

Interesting to note that the old Italian and English car manufactures figured out this problem 50 years ago. Those old cars leaked out oil faster than it could deteriorate. If you had to add a quart every week or two it was a similar to a continuous oil change!

Hmmmmmm................That explains Corvairs. There was some VW/Porsche influence there.

Anybody remember the Frantz? Changing the TP fairly often gave the engine a new quart of oil, doing, in effect, a British Oil Change.

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I do not agree with either of these statements as applied to vehicle engines. Most all of the performance engine builders agree that a minimum of 10 PSI/1000 RPM is need to insure lubrication and prevent engine failure. While pressure isn't the real factor, it is needed to insure that the film of oil between various rotating metal parts is thick enough to provide a barrier and prevent metal to metal contact under ALL circumstances. Turbo charged engines or very high compression engines present more of a lubrication challenge as well.Any brief loss of pressure/oil volume or air in the system will and does cause bearing failure almost immediately.

Well, it's true. Ideal lubrication is based on quasi-hydrodynamic flow , and the demands on the lubricant are less at higher RPM (just as an aircraft needs less angle-of-attack at higher speeds or a hydroplane has littel wetted area at high speeds). Your assumption is correct in that there needs to be adequate pressure to ensure cooling flow, which can be as important as lubrication.

As an aside, the functions of a motor oil are:

Lubricate
Cool
Clean
Seal
Protect

quote:

Originally posted by Rusty:
Your assumption is correct in that there needs to be adequate pressure to ensure cooling flow, which can be as important as lubrication.

Olds cranks had a larger rod journal, and, in jet boat service, would overheat the oil there. Milling a slot on each side of each rod and its cap would let enough extra oil out to keep the bearing oil cool enough to live.

My knowledge in this area is somewhat dated but the basics probably have not changed much.
The original applications for synthetics was, not surprisingly military. Where else could you pedal an oil for $4 to $5 per gallon in 1968 when Rotella was selling for about $1.70/gallon? The application was jet engines. Viscosity for the Navy 5 cs. and the Air Force 3 cs. where one centistoke (cs) is about the viscosity of water. (Navy felt it could keep its planes warmer than the Air Force) Adjustment to achieve the final viscosity was done with addition of a polymer, think STP. Because the operational speeds of the shaft would be 10,000 rpm plus the viscosity need not be high since the support of the fluid increases with rotational speed. A fluid is dragged along by the moving shaft and builds up resistance to being displaced. At some speeds air bearings are employed. If the fluid is too thick it will not rotate as well, causing drag. At low speeds one of the oil additives is able to attach to the bearing metal and support the shaft or prevent it from scoring the bearing during its increase in speed. This is true for all oils and applications. This is the basis for the old saw about the greatest wear being in the first several seconds.
The original oils I worked on that were synthetics were very clean and cleaning. Whether this second trait is or is not a natural property, I don't know but we experienced bearing failures due to seals being cleaned of deposits.
My feeling is that unless you are using something 24/7/365 or very nearly so the current quality oils are a very good economic choice, not so different from those many years ago.
FWIW,
Tim

Thanks for the great comments. Looks like I'll stick with Rotella.