The following information was submitted by KwakNut, a member who works at a high level in the oil industry:


"About synthetics:

First, a lot of the myths about synthetics go back several decades when the original ‘synthetics’ were strange indeed, such as glycol based, and had very different properties to your normal mineral oil – there were problems with them attacking seals, they had disadvantages for every advantage, and you absolutely could not ix the with mineral oil.  However, there are many kinds of synthetics.

Modern polyalphaolefin ‘synthetics’ we use in retail automotive applications are not like that – they’re 100% compatible with mineral oils because they are the same compounds, just made in a purer way than nature can do it.



What people fail to grasp is that these synthetics are chemically the same as mineral oils, just purer.  Your average engine oil is made up of molecules each consisting about 30 carbon atoms, with obviously a few bits of oxygen and hydrogen thrown in.  In a mineral oil, they are going to be in all sorts of shapes -  think of it like the variation in shape you get with snowflakes.  The odd one will just be a neat, perfect, straight carbon chain – but not many at all.  Those others, the majority of randomly-shaped 30-weight molecules all have physical weaknesses, and ‘corners’ where other chemicals find it easier to react and attach.  



In your traditional mineral oil/liquefied dinosaur juice, it all starts out as one thick gloop that comes out of the ground and we fractionally distil it to get different ‘weights’ out at the refinery.  The thin stuff goes in fuel tanks, the medium stuff is lube, and the thick stuff makes roads.  Vacuum distillation separates the oils in to different weights of base stock – hence the 30-ish carbon molecule size in our oils.  



A true synthetic engine oil is polymerised – made up from C2 gas (ethane), to produce perfect straight carbon chains.  They are chemically identical to the less smoothly shaped natural molecules, they have the same chemical formula, but they are far more stable, not just to oxidation and chemical breakdown, but to physical duress as well – in gear teeth under extreme pressure, those molecules just slip past each other rather than breaking each other’s arms off.  That results in many advantages:



The oil remains more stable, doesn’t break down as quickly, and lasts far longer in the sump; less friction heat is generated, the oil itself exerts less friction against the metal – so less wear; less energy is used to turn the machine because of that reduce friction – so you actually gain in power and economy.  I could go on here.



Think of the oil molecules as being like components that come off a rough production line.  The very best items get inspected and put to one side for custom and aerospace applications – the rest goes into cheap mass production.  Well, with these fully synthetic oils, it’s like every molecule you get is the individually inspected blueprinted component you’d get from a custom machine shop, while normal oils contain all the average stuff and all the rejects too.



In short, it’s the same as mineral oil, but purer – much purer.



Additives

Engine oils contain a complex additive package.  Key to internal combustion engines is the dispersant/detergent pack.  The detergent coats metal surfaces to stop carbon waste products from adhering, while the dispersant is made up of little charged molecules which attach to small pieces of carbon and cause them to repel from each other – that way, the particles of suspended carbon (which are going to be there from fuel combustion and a little from oil burned on the bores whether you want it or not) always stay smaller than the oil film, and do not rub against your engine’s precious working surfaces.

There are also anti-wear agents, similar to those used in gearboxes which are activated by the temperatures caused at local pressure points, and effectively put a protective chemical coat on the metal, and there are viscosity improvers, which are spiral molecules which expand when hot, helping to counteract the thinning of an oil with increased temperature.

There are also emulsifying agents; their job is to trap water, in an emulsion, which allows the oil to absorb, safely, a certain amount of crankcase condensation or other moisture ingress.  Synthetics do not have any ‘problems’ with condensation that other oils would not have, whatever anecdotal evidence may have been observed by the odd individual.



Additives are important in engine oil – what we all need to remember is that you get what you pay for, and the good additive packs don’t go into cheap oil.  The way those additives perform comes down to the additive pack, not the oil base stock (though a good base stock will need the assistance of the additives less than a poor one).




Viscosity of synthetics:




Firstly, what do we mean when we say 10W40 or 20W50?  The first number, the W rating, says how thick the oil is at zero centigrade.  The second number says how thick it is at 100 centigrade.  You’ll notice that a mineral oil will probably be 15W40 or 20W50, while the synthetic can be 0W40 or 5W50.  That’s because the synthetics have a flatter viscosity index – and that means that their viscosity varies less with temperature.  That’s partly down to the expensive additive pack, partly lack of waxy impurities, but mostly the purity of the base stock.  



On a zero centigrade/32F cold morning, a 10W40 mineral oil will be the same thickness as a 10W50 synthetic – same pressure reading when you start up, and similar at room temperature.  However, when it gets really cold, like -15C/5F or worse, your mineral oil gets really thick, because it just doesn’t like that cold, and until it warms up your engine struggles to pump it round – it can be several seconds on that kind of cold day before the top end gets any oil at all!  With the synthetic, that flat viscosity index, and lack of waxy impurities, means that there is far, far less thickening when it gets really cold, and the oil still flows easily.



You can test this yourself.  Take an egg cup (or a smaller container) of Mobil 1, and of a similar W-rated mineral oil, and leave them in the deep freeze overnight.  See how they pour in the morning.



When it gets hot, obviously the reverse happens.  The 5W50 synthetic is the same thickness as the 20W50 mineral oil at 100C, but when things go wrong the viscosity of the mineral oil drops right off, and it starts to evaporate too quickly off hot bores.  The synthetic retains much more viscosity than the mineral oil, doesn’t evaporate off the bores as easily and remains far, far more resistant to thermal breakdown.  In Europe GM launched a car called the Vauxhall Carlton some 20-odd years ago.  Lotus had the contract to develop it from a 3-litre, 4-door saloon to be ‘quicker than a Ferrari Testarossa’.  They achieved it, but struggled badly to get the 377bhp they needed from that 3 litre turbocharged engine (this was the 80s remember) because they couldn’t keep temperature down at the boost they needed, and the oil was boiling in the sump.  The answer?  Mobil1 5W50, and thick black print in the owner’s handbook not to use any other oil!



Again, you can test it yourself.  Place a teaspoon each of 20W50 mineral oil and 5W50 synthetic in a baking tray in a hot oven, and see how they take the heat (best to do this when the mrs is out)





You’ll always here people come out with stories of how they’ve had problems with synthetic oil.  What they can rarely tell you is whether they’d have had the same problem with mineral oil, or if they just hadn’t noticed up to the point they changed the oil!  

Lots of people will continue to resist the expertise of some pretty clever people who develop these products for the oil companies, and go with the advice of the mechanic at their local bar who wouldn’t even trust synthetic oil in his lawnmower.  He’s like a fellow in my local pub who won’t wear a seatbelt in case he can’t duck to one side when a truck full of scaffolding poles gets spilled over his hood – he’d get skewered if he wore his belt, so he’s convinced that it’s safer to drive without a seat belt on.  I doubt he really has to worry about being run through by scaffolding poles, and I just hope he doesn’t have a regular crash."


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