Why does synthetic cause leaks in older engines?
Cheers
Simon
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Early, and poor quality mineral oils used to swell seals because of the light fractions they included, some seals could then wear in their swollen state, when a vehicle was switched over to synthetic, these compounds leached back out of the seals and they would shrink and leak. Modern seals are made from different material, and modern mineral oils contain less of these compounds.
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One of the biggest misconceptions (and there are many) regarding synthetic oils is "they are too thin".
It is a FACT that it is not the basestock (petroleum/synthetic) that dtermines the viscosity (thickness) of the oil - IT'S THE VISCOSITY
Example:
5w-40 is thinner than 15w-40 BUT only when it's cold, it is in fact the same SAE 40 when it's hot.
All oils whether synthetic or petroleum need to pass the API viscosity test to be rated as a multigrade so there is no difference.
The answer is to use the correct viscosity - whether you choose to use mineral, semi-synthetic or synthetic is down to cost at the end of the day but, quality wise you get what you pay for and synthetics are superior lubricants.
Cheers
Simon
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Simon
Yes, I've never understood this idea that synthetics cause leaks, for precisely the reasons you describe.
It's probably worth nailing one of the other misconceptions too. All oils, including multigrades, reduce viscosity (get 'thinner') when they are hot. So despite the fact that, say, a 5W-40 seems thin when it's poured from the can, it's still considerably less viscous when it's hot, even though it is then behaving as an SAE40. So, the lower viscosity of the cold oil won't cause leaks. In fact, by potentially reducing peak oil pressures when cold, it could actualy reduce leakage rates.
JS
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All oils, including multigrades, reduce viscosity (get 'thinner') when they are hot. So despite the fact that, say, a 5W-40 seems thin when it's poured from the can, it's still considerably less viscous when it's hot, even though it is then behaving as an SAE40.
JS
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john s - did you mean
a 5W-40 seems thin when it's poured from the can, it's still considerably less viscous thanwhen it's hot, even though it is then behaving as an SAE40.
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Dalgliesh
No, quite the opposite. I didn't make it very clear. 5W-40 seems thin when it's poured from the can but it's considerably MORE viscous when cold than when it's hot.
All oils become less viscous with increasing temperature, it's just that multigrades thin out less than monogrades.
JS
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Aside from anything else you say about oil, I'm struggling with this sentence;
"that determines the viscosity of the oil - IT'S THE VISCOSITY"
Essentially "viscosity determines the viscosity" ? Or my weight determines my weight ?
Am I missing something ?
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Mark, I think it's a figure of speech, like "As sure as eggs are eggs"
Viscosity is viscosity, and not something contrived in the mind from other attributes such as sythetic or mineral.
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With permission, I quote from a recent post on the MB Owners Club Forum, from Chris. Note that it does refer to a 1986 W123:
"My car has a main dealer service history from new and during that time has never had synthetic oil in it - except for one occasion. The dealer put in fully synthetic by mistake and within 100 miles the stuff was coming out from gaskets all over the place, much to my annoyance. I took it back and they checked their records and confirmed the mistake and put in the usual Shell mineral oil and within another 100 miles all was well again."
That's consistent with all the warnings and advice I've seen from experts, especially in relation to older cars, but equally valid I would think for younger cars that have been accustomed to mineral oil for many tens of thousands of miles.
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Hi,
Viscosity is merely (merely?) a measurement of how a fluid will flow, so it doesn't matter if it's honey, oil, water or whatever. Or mineral or synthetic motor oil, for that matter. The only external agent that affects viscosity is temperature. Fluids in general, and motor oils in particular, will get thicker when cold and thinner when hot. Viscosity is therefore meaningless without temperature (OK, except for water, which has a fairly constant viscosity as long as it's not solid or steam of about 1 cSt).
A fluid's ability to flow is measured by kinematic viscosity, expressed in centistoke (cSt), with lower ratings, 30 cSt for instance, being thinner (less viscous) than 40 cSt. A motor oil's viscosity however (a multi-grade motor oil, that is), is graded in terms of both flow and absolute viscosity, and these are measured at very different temperatures. So a 10W-40 oil is not thin when cold and thick when hot. Far from it.
When an engine is hot it needs the oil to flow quickly and easily around the galleries and to the bearing surfaces. So the running temp ratings, the higher 30, 40 and 50 grades, refer to kinematic viscosity (cSt), how easily an oil will flow. They are all measured at a notional running temperature of 100 deg C, and mean that the oil is within a certain viscosity range at that temperature. It will certainly be very thin as well as very hot. The viscosity range for each oil grade is quite wide, and oils with the same specification from different manufacturers may vary considerably in viscosity and still conform to the grade. The exact viscosity, in cSt, can be found for most popular oils in literature or on the internet.
When an engine is cold it needs to be able to be turned over and it's oil to be fluid enough to be pumped. So the cold or 'Winter' specification, the lower 5W, 10W etc. grade, refers to absolute viscosity related to cold cranking and cold pumping capability, measured by rotary viscometers and expressed in centipoise (cP). It is not a cSt rating and can't be directly compared with the hot flow rating. The pumping viscosity specifications for 0W, 5W, 10W, 15W, 20W and 25W grades are all exactly the same. The difference is the temperature they're measured at: 25W at -15 deg C, 20W at -20 deg, 15W at -25 deg, 10W at -30 deg, 5W at -35 deg, and 0W at -40 deg C. Truly viscosity is meaningless without temperature. These pumping viscosities are measured at a temperature 5 deg lower than the corresponding cranking viscosity, on the grounds that if the engine will turn over and start, the oil must be capable of being pumped.
As the specification for 40 grade oils (for instance) is constant for all oils at 100 deg C, the cry that synthetic oils are 'too thin' doesn't stand up, not at running temperature anyway. All 40 grade oils must have a viscosity between 12.5 and 16.3 cSt at 100 deg C whatever their composition. I think that this 'thin oil' misconception comes from synthetic oils generally having lower winter ratings and thus appearing to be, and actually being, thinner at ambient temperature than the older mineral grades that some may be used to. There is also the understandable conception that a thicker oil will protect the engine better, which is not necessarily the case. All this measurement, of course, takes place when the oil is new. Lower quality oils may achieve a wide cold/running grade spread with a higher amount of VI improver polymers, which may degrade fairly quickly in use.
Rgds.
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OK. Now what about the widely touted opinion (on which I have no view by the way) that synthetics, being supposedly more "slippery" than dino oil, cause roller bearings to "skate" rather than "roll" and thus develop flat spots.
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I have another qustion. What is synthetic oil? What is it made from if not from mineral oil? I'm puzzled about the name 'synthetic' as most synthetic products are made from mineral oil anyway.
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Hi,
I am not a chemist, or employed in the oil industry. I'm just an interested passer-by.
Synthetic oil is more or less just what you want it to be. Possibly the best use of the word synthetic is to make us feel better about coughing up exhorbitant UK prices for a marketing label.
In the old days (in the early 70's) synthetic oils were made from Group IV base stocks, the polyalphaolefins (PAO). Then Group V base stocks came along, the Esters, and true synthetics were a mixture of these two. The PAOs are synthesized from ethylene, according to Mobil, and this is a product of crude oil distillation. Ethylene is a two-carbon molecule, and Mobil synthesises it into 10-carbon molecules, and combines three of these into PAO, a very stable, clean and controllable product. Even in those heady days Mobil 1 (possibly the most documented oil there is) wasn't 100% synthetic, as the carrier oil (up to 20%) which contained the additive package was a mineral oil.
All the mineral oils had to get by with were Group I and II base stocks. The Group I's were very cheap and cheerful, and the Group II's were good quality and hopefully what we used in our vehicles. Then, in the 90's, Castrol with its Syntec - formerly a PAO - decided that it could use Group III base stocks from Shell instead at a significant cost saving.
Group III base stocks are hydroisomerized (hydrocracked) mineral oils. Mineral oils are composed of mainly carbon and hydrogen. Ideally the carbons are sequentially linked to one another to form long chains. Less stable forms include branches or rings, and contamination with sulphur or nitrogen atoms. These forms are highly undesirable. Surrounding the carbon chains are hydrogen atoms forming a protective barrier. Ideally each carbon atom would have two hydrogen atoms bonded to it, with an additional hydrogen atom topping and tailing the chain at each end. The molecules would thus be fully saturated.
Mineral oils are stabilised and cleaned of sulphur and nitrogen by solvent extraction, a washing away of some of the unsaturated oils which is ineffecient and only around 90% effective. Hydrocracking is a process that involves heating the oil to 700 deg F at 3000 psi in the presence of hydrogen and a catalyst, in stainless steel reactors with walls over a foot thick. The oil molecules become fully saturated, away goes sulphur and nitrogen, and carbon rings are opened into chains. Now you have a 'synthesized' mineral oil. A very high quality stable synthesized mineral oil.
Mobil complained about Castrol's synthetic advertising and their lack of synthetic content to the National Advertising Division of the Council of Better Business Bureaus in the USA. Mobil said that synthetic means small molecules synthesised into larger ones. Castrol said it meant the product of an intended chemical reaction. NAD agreed with Castrol.
The only response left to Mobil was to use some Group III hydrocracked oils in their Mobil 1 Tri-Synthetic formulation, and that's what they did. As far as I can tell Mobil 1 is composed of Group IV and V stocks (the PAO and Esters), a further synthetic ester and an alkylated aromatic fluid. An alkylated aromatic is a 'synthetic hydrocarbon.' A mineral oil.
I don't know any commonly available oil in the UK which is fully synthetic as Mobil would like to see it defined. There are many very fine oils which will give superb protection whatever the label. Personally I use Mobil 1 in my MG and a semi-synth (now there's a story) in the Rover. I'm happy with them all.
I'm sorry if this is a little heavy. There's reams more of it, I'm afraid.
Rgds.
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I've put fully-synthetics in old Mercedes engines and never had a problem...
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On one of the expert websites that I visited recently, the verdict was that semi-synthetics are a waste of money, as they show no performance improvement over pure mineral oils, in spite of costing considerably more. Are they profit enhancers, rather than performance enhancers -- giving the consumer the illusory comfort of being "better" than mineral oil and less "high-tech risky" than the more costly fully synthetics?
The author was also convinced that you get what you pay for, so the top-flight synthetics are not only the best lubricants but also worth every penny.
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One of the causes of sludging in certain VW engines seems to be the use, in the USA particularly, of mineral oils rather than the semi or fully synthetic oils to VW-inhouse standards required by VW. The oil change intervals for the engines were based on a good quality oil: running for those intervals (or longer in some cases) seemed to cause failure. So, there does seem to be a performance difference between mineral and semi-synth. It may be the total package rather than the oil-base thats influential: I'll end on this speculative note, as I am no expert.
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Hi, Viscosity is merely (merely?) a measurement of how a fluid will flow, so it doesn't matter if it's honey, oil, water or whatever. Or mineral or synthetic motor oil, for that matter. The only external agent that affects viscosity is temperature. Fluids in general, and motor oils in particular, will get thicker when cold and thinner when hot. Viscosity is therefore meaningless without temperature (OK, except for water, which has a fairly constant viscosity as long as it's not solid or steam of about 1 cSt).
An oils viscosity can also be affected my mechanical influences. Some oils "shear thin" - the viscosity reduces when it is pumped. This obviously can be important in an engine.
Joe
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Absolutely right. I'd say that all, not some, oils suffer from viscosity reduction due to shear in service. It's one of the aging effects on engine oils. The better quality oils withstand this better than the cheaper basic oils.
JS
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Hi,
Yes, you're right, oils (or any fluids) can have a different viscosity rating when they have been subject to mechanical and chemical operations. The point was that viscosity is a specific measurement at a particular time and temperature. Add petrol, soot, acids, metals, etc. and it will change.
I dunno about oils thinning in use. I guess some do, but perhaps even more don't. Most motor oils tested under very arduous load and temps tend to turn into treacle, but perhaps those tests are unrealistic. Even Mobil 1 in the 18,000 mile test at neptune.spacebears.com/cars/stories/oil-life.html thickened a little, whilst Amsoil went from a 5W-30 to a 15W-40 over 14,000 miles.
What's fascinating is why so many people find talking about oil so fascinating.
Rgds.
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Indeed it's a fascinating subject and one that I personally enjoy talking about.
ALL oils thin/lose viscosity (shear) with use.
It's the rate of shear that's important and this is where Group IV and V basestocks are better. The primary reasons for shear in a multigrade are the Viscosity Improvers that are added.
These are polymers that are temperature sensitive which uncoil with heat to assist the oil in "fighting" the thinning effect caused by the higher temperatures.
Unfortunately it's these that shear and therfore lose their ability to "fight back" for want of a less technical term.
So all that being said, these VI improvers not only vary in quality but also vary in the amount added to the base oil and therefore, less is better and higher quality is better.
The wider the range a multigrade covers (10w-60 compared to say 5w-40 = 50 vs 35) the more of these VI Improvers need to be added and therefore the more prone the oil is to viscosity loss (shear) with use.
The reason that Group IV and V synthetics are better is because they require little or none of these VI Improvers to operate as they are more thermally stable to start with.
The main reason for this is that they are not refined to remove impurities they are actually built by chemists in laboratories specifically to fit an application. This means that they have molecules of a uniform size as opposed to various sizes.
As to the question of them being worthwhile, well they are certainly superior lubricants but it depends on the application and the frequency of oil changes you require.
These oils last longer and cost more and are always adviseable in stressed applications where they will outperform and out live their petroleum cousins by a large margin.
Cheers
Simon
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