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Slippery Stuff - All About Oil

 by pirateparts on 28 Feb 2012 |
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A Very Slippery Subject – Oil You Need to Know

Few topics arouse as much passion in petrolheads as that of engine oil.  People cling to articles of faith about their favourite oil with the same degree of strength, certainty and fervour as TV evangelists trying to pry open wallets.  There are three things that you need to know about an oil in order to make an informed choice:
1.       How good is the oil?  This question is answered by its API rating.
2.       How well does the oil flow?  This question is answered by the viscosity rating.
3.       What is the oil made of?  This question is answered (sort of) by whether the oil is a mineral oil, a synthetic oil or a halfway house, neither fish nor fowl semi-synthetic oil.
Let’s take a closer look at each of those indicators in turn.
 
API Ratings
Over the years oils have improved enormously.  One of the main areas of change over the years has been the additive pack, the detergents and other chemicals that oil companies add to their oils to reduce wear, carry acids and other by-products of combustion and generally make engine oil the useful stuff that it is.  API ratings for spark combustion (i.e., petrol) engines are S-ratings.  The current rating is SN, but you will see older ratings such as SM, SL, SJ or even SG still around.  The API rating is really the only unambiguous indicator of how good an oil is, although it doesn’t quite tell us everything that we need to know.
 
Going With the Flow - Viscosity
This section is a little more complicated, but I will try and be brief.  As well as having a pleasing golden colour, motor oil shares other characteristics with honey.  For one thing, it is a little known fact that bees manufacture excellent motor oil.  OK, I made that bit up.  Here’s the real story: like honey, oil flows better when it is warm than when it is cold.  Most of Australia has a mild to warm climate, which means that we do not need to worry about the oil in our car’s engines turning super-gloopy in winter.  Even so, oil flows better when it is warm than when it is cool.  This is one of the reasons why fanging a cold engine is bad for it – the relatively gloopy oil does not create a suitable film between moving parts, which leads to wear.  Which is also why you will often hear the claim that most engine wear happens at start-up.
 
In the early days of motoring, engine oils were monogrades.  So an oil might have a viscosity of SAE10, which would watery be like 3-In-One oil, or SAE40, which would flow a lot slower.  As oils warm up, they flow better as they thin out.  So a monograde oil with a viscosity of SAE10 might flow nicely when cold, but might thin out so much when warm that oil pressure would drop off.  And an SAE40 oil might be great at maintaining pressure when warm, but when cold, would not flow enough to lubricate properly.
 
Enter multigrade oils.  An oil with a viscosity rating of 0W40 flows easily when cool (that’s the 0 part), but remains viscous enough to maintain good pressure when hot (that’s the 40 part).  In other words, it has the viscosity of an SAE0 oil when cool, but an SAE40 oil when warm.
 
Oil pressure is a lot like porridge temperature – there’s a “just right” amount.  Not enough oil pressure doesn’t get oil to where it needs to be.  Too much oil pressure pops oil seals.  The right amount of oil pressure ensures that oil gets where it needs to be, but it’s oil flow and not oil pressure that really does the work of lubricating the moving parts of an engine.  And oil flow and oil pressure are usually inversely related: more pressure means less flow, less pressure means more flow (all other things being equal).
 
So what viscosity should you be using?  Well, you should be using the thinnest hot viscosity oil that delivers the oil pressure that your engine requires.  And as thin when cold as will stay in the engine.  I’ve found that very thin oils (and I am talking about the cold viscosity here) have a way of sneaking past seals and gaskets, especially in older cars.  So, to re-cap, a 0W40 oil and a 10W40 oil will flow exactly the same at operating temperature, but the 0W40 oil will flow better when cold than the 10W40.  The downside is that the 0W40 might be able to find escape routes from your engine.  I guarantee that you will hear people telling you that a 0W40 doesn’t offer the same protection as a 10W40.  They are just plain, flat-out wrong.
 
Synthetic vs. Mineral Oil
The oil that bubbles up from under the ground is the mortal remains of dead animals, usually plankton and algae formed over millions of years of being compressed under sedimentary rock.  This is crude oil, the basis for petrol and mineral engine oil (among other things).  Clever chemists found ways to manufacture synthetic oils in the laboratory.  Marketers persuaded us that synthetic oils were better than mineral oils, and by and large they are right as synthetic oils are better at not breaking down under high temperatures than mineral oils.  This is what makes synthetic oils a good choice for motorsport applications.
 
More recently, a court case found that synthetic oils can be manufactured from mineral oil base stocks, and everything got confusing all over again.  Which brings us back to API ratings as the only definitive measure of how good an oil is.
 
Which oil should you use?  In a modern car, you should use the highest API rated oil you can find, and as runny an oil (i.e., a low viscosity rating) as will deliver the required amount of oil pressure.  Use a synthetic oil if your engine manufacturer recommends it, or if you are spending time on the race track.

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All pictures and references to the brands and logos are for reference only, and do not imply any association with the brandholders. Most parts offered are OEM or quality aftermarket parts.  Genuine Porsche or MINI parts are specifically identified as such.  Master Parts  is not responsible for any typographical errors contained within the site.   Information within this website is for reference only.   It is your responsibility to verify that you are technically competent to carry out repair and maintenance procedures.  By entering this site, you agree to hold Master Parts free from any liability arising out of the use of any information contained within.
 
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