A Guide to Engine Oil Specifications

Lubricating the High Performance Engine

A Guide to Engine Oil Specifications

A wide variety of different engine oils with different specifications is available.

This guide explains what the most important engine oil specifications mean:

API:

API (American Petroleum Institute) categories refer to American requirements and quality criteria which engine oils must fulfil.  The current API grades are SM for petrol engines and Cl-4 for commercial diesel engines.  Car diesel engines are not currently classified by API

SAE:

The flowing properties of an engine oil are described by SAE (Society of Automotive Engineers) grades.  Taking the example of an SAE 0w/30 engine oil, the first numeral describes the viscosity at low temperatures.  The lower this first number, the better the oil flows at low temperatures to reach the important lubrication points in the engine.  The second figure describes the viscosity at high temperatures.

Petrol Engines

Car Diesel Engines

Commercial Vehicle Diesel Engines 

Manufacturer’s Approvals

Apart from the specifications listed above, a number of manufacturers demand tests on their own engines.

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This article is quite long but well worth the read.

Lubricating the High performance engine

Basically

Basically, to use that irritating in-word, engine lubrication is simple, and consequently boring.  So I intend to treat the subject ‘complicatedly’, which may not be an in-word, but makes life far more interesting!

So, to take a quick look at the simple picture; the oil keeps moving parts apart, reducing friction and carrying away heat.  Where there is metal-to-metal contact there are chemicals in the oil to reduce the damage.  Because the internal combustion process is always less than perfect, some soot is produced and this must be washed off the pistons and rings by the oil, so it has a cleaning or detergent function.

The trouble is, all this is just as true for Henry Ford’s original Model T engine as it is for any high performance engine.  So where is the difference?  The Model T, with 10b.h.p. / litre at 2,000 revs per minute and a single underhead camshaft, was filled with a thick, greenish liquid from somewhere near the bottom of the distillation columns on the Pennsylvania oilfields.  It did a vague tour of the internals by guesswork (there was no oil pump) at a temperature around 50° C, and lasted for 1,000 miles.  On the plus side, some of the impurities acted as anti-wear and detergent chemicals.  They didn’t work very well, but it was better than nothing.  The engine wore out in around 20,000 miles, but even ordinary people, not just amateur race and rally drivers, were happy to put up with this.

The difference begins with the first turn of the key.  The modern high-pressure pump would cavitate on the old heavy monogrades, starving the bearings for a vital couple of seconds, even in warm weather.  Likewise, cam lobes would suffer as the sluggish oil found it’s way along narrow oil ways to the valve gear.  The turbo bearing (‘if fitted’, as the handbook says) already spinning fast, would also starve, and when it got going, how long would it be before the heat soak-back fried the primitive oil into a lump of carbon?  (This was a problem even with ‘modern’ oils only 20 - 25 years ago.)

So, a good oil must be quite low in viscosity even in the cold, so that it gets around the engine in a fraction of a second on start-up.  On the other hand, it must protect engine components (piston rings for example) at temperatures up to 300° C without evaporating or carbonising, and maintain oil pressure.

Unmodified thin oils simply can’t manage this balancing act.  The answer is to use a mixture of thin oil and temperature-sensitive polymer, so as the thin oil gets even thinner with increasing temperatures as the engine warms up, the polymer expands and fights back, keeping the viscosity at a reasonable level to hold oil pressure and film thickness on the bearings.  This is called a multi-grade.

But this is all too basic!  What I have just written was and is relevant to a 1958 Morris Minor.

The questions that high performance engine owners need to ask are “Will this thin base oil evaporate and be drawn into the intake manifold (via the closed circuit crankcase ventilation), leading to combustion chamber deposits and de-activated catalysts?” and “Will the polymer shear down at high engine revolutions and high temperatures, causing low oil pressure and component wear?”  and “Will it carbonise on the turbo bearing?”  These are 21^st century questions which cannot be answered by a basic 1990’s approach.

BUT!  Before we head into more complications, some figures…

The SAE Business (American Society of Automotive Engineers)

Viscosity is the force required to shear the oil at a certain speed and temperature.  Oils work because they have viscosity; the drag of a rotating part pulls oil from a low-pressure area into a high pressure area and ‘floats’ the surfaces apart.  This is called hydrodynamic lubrication, and crank bearings depend on it.  In fact, a plain bearing running properly shows literally no metal-to-metal contact.  Experimental set-ups have shown that electrical current will not flow from a crank main bearing to the shells.  Also, the energy loss due to friction (the co-efficient of friction) is incredibly low, around 0.001.  So for every kilogram pulling one way, friction fights back with one gram.  This is very much better than any ‘dry’ situation.  For example, the much over-rated plastic PTFE has a co-efficient of friction on steel of 0.1, 100 times worse than ‘ordinary’ oil.

Oil viscosities are accurately measured in units called ‘Centistokes’ at exactly 100° C.  These fall into five high temperature SAE categories:-

A decent quality oil usually has a viscosity that falls in the middle of the spec, so a SAE 40 will be about 14 Centistoke units, but SAE ratings are quite wide, so it’s possible for one 40 oil to be noticeably thicker or thinner than another.

When the polymer modified multigrades appeared, a low temperature range of tests were brought in, called ‘W’ for Winter (no, it doesn’t mean Weight!).  These simulate cold starts at different non-ferrous monkey endangering temperatures from -15º C for the 20W test to a desperate -35º C for zeroW (0W).  So, for example, a SAE 5W/40 oil is one that has a viscosity of less than 6600 units at –30º C, and a viscosity of about 14 units at 100º C.  Now, those who have been paying attention will say “Just a minute!  I thought you said these multigrade polymers stopped the oil thinning down, but 6600 to 14 looks like a lot of thinning to me!”  Good point, but the oil does flow enough to allow a marginal start at –30º, and 14 is plenty of viscosity when the engine is running normally.  (A lot more could damage the engine, so we don’t recommend the 24 viscosity SAE 60 oils.)  The vital point is, a monograde 40 would be just like a wax candle at –30º C, and not much better at –10º C.  It would even give the starter motor a fairly difficult time at zero.  (At 0º C, a 5W/40 has a viscosity of 800 but the mono 40 is up at 3200.)

Another basic point about wide range multigrades such as 5W/40 or 0W/30 is that they save fuel at cruising speeds, and release more power at full throttle.  But complications arise…

Building a good oil

A cave may not be the best place to live, but it’s ready-made and cheap.  This is the estate agent’s equivalent of an old-style monograde oil.  Or you could get Hengist Pod to fit a window and a door; this is moving up to a cheap ‘n’ cheerful mineral 20W/50.  But an architect-designed ‘machine for living in’, built up brick by brick, is an allegory of a high performance synthetic oil

It is impossible to make a good 5W/40, or even 10W/40, using only mineral oil.  The base oil is so thin, it just evaporates away at the high temperatures found in a powerful engine that is being used seriously.  Although there are chemical compounds in there to prevent oil breakdown by oxygen in the atmosphere (oxidation) they cannot adequately protect vulnerable mineral oil at the 130º C plus sump temperatures found in hard worked turbocharged or re-mapped engines.

Synthetics are the answer.  They are built up from simple chemical units, brick by brick so to speak; to make an architect-designed oil with properties to suit the modern engine. 

But sometimes, if you look behind the façade, there is a murky old cave at the back!  This is because the marketing men have been meddling.

The Synthetic Myth

What do we mean by the word ‘synthetic’?  Once, it meant the ‘brick by brick’ chemical building of a designer oil, but the waters have been muddied by a court case that took place in the USA a few years ago, where the right to call heavily-modified mineral oil ‘synthetic’, was won.  This was the answer to the ad-man’s dream; the chance to use that sexy word ‘synthetic’ on the can… without spending much extra on the contents!  Most lower-cost ‘synthetic ‘ or ‘semi-synthetic’ oils use these ‘hydro-cracked’ mineral oils.  They do have some advantages, particularly in commercial diesel lubricants, but their value in performance engines is marginal.

True synthetics are expensive (about 6 times more than top quality mineral types).  Looked at non-basically, there are three broad categories, each containing dozens of types and viscosity grades:-

1)      PIB’s (Polyisobutanes)

These are occasionally used as thickeners in motor oils and gear oils, but their main application is to suppress smoke in 2-stiokes.

The two important ones are:-

2)      Esters

All jet engines are lubricated with synthetic esters, and have been for 50 years, but these expensive fluids only started to appear in petrol engine oils about 20 years ago.  Thanks to their aviation origins, the types suitable for lubricants (esters also appear in perfumes; they are different!) work well from –50º C to 200º C, and they have a useful extra trick.  Due to their structure, ester molecules are ‘polar’; they stick to metal surfaces using electrostatic forces.  This means that a protective layer is there at all times, even during that crucial start-up period.  This helps to protect cams, gears, piston rings and valve train components, where lubrication is ‘boundary’ rather than ‘hydrodynamic’, i.e. a very thin non pressure-fed film has to hold the surfaces apart.  Even crank bearings benefit at starts, stops, or when extreme shock loads upset the hydrodynamic film.  (Are you listening, all you rally drivers and off-road fanatics?)

3)      Synthetic Hydrocarbons or PAO’s (Poly Alpha Olefins)

These are, in effect, very precisely made equivalents to the most desirable mineral oil molecules.  As with esters, they work very well at low temperatures, and equally well when the heat is on, if protected by anti-oxidants.  The difference is, they are inert, and not polar.  In fact, on their own they are hopeless ‘boundary’ lubricants, with less load carrying ability than mineral oil.  They depend entirely on the correct chemical enhancements.

In fact PAOs work best in combination with esters.  The esters assist load carrying, reduce friction, and cut down seal drag and wear, whilst the PAOs act as solvents for the multigrade polymers and a large assortment of special compounds that act as dispersants, detergents, anti-wear and anti-oxidant agents, and foam suppressants.  Both are very good at resisting high-temperature evaporation, and the esters in particular will never carbonise in turbo bearings even when provoked by anti-lag systems.

Must Have MORE Power!

Motorcars are bought for all sorts of reasons, but enthusiasts like lots of power.  To get power, a lot of fuel must be burnt, and more than half of it, sadly, gets thrown away as waste heat.  For every litre of fuel burnt, 60% of the energy goes as waste heat into the exhaust and cooling system.  A turbocharger can extract a few percent as useful energy and convert it into pressure on the intake side, but only 40-45% is left, and only 25-odd percent actually shows up as B.H.P. at the flywheel.  6% goes in pumping air into the engine, 6% as oil drag losses and 2 or 3% as engine friction.  The oil deals with at least 97% of the friction; so reducing the remaining few percent is not easy.  If you doubt that even an ordinary oil has this massive effect, take a clean, dry 200 BHP engine, connect it to a dyno and start it up.  It will only make 1 BHP for a few seconds.  Now that’s real friction for you! 

Oddly enough, people get starry-eyed about reducing friction, especially those half-wits who peddle silly ’magic additives’, which have not the smallest effect on friction but rapidly corrode bearings and wallet contents.  In fact, even a virtually impossible 50% reduction in the remaining engine friction would be no big deal, perhaps one or two BHP or a couple of extra miles per gallon.

Even More Power!

The place to look for extra power is in that 6% lost as oil drag.  In a well-designed modern motor, the oil doesn’t have to cover up for wide clearances, poor oil pump capacity or flexy crankshafts, so it can be quite thin.  How thin?  Well, take a look at these dyno results.

Some time ago we ran three Silkolene performance oils in a Honda Blackbird motorcycle.  This fearsome device is fitted with a light, compact, naturally aspirated 1100c.c. engine which turns out 120+ BHP at the back wheel.  The normal fill for this one-year-old engine was Silkolene Turbolene GTI 15W/50, so the first reading was taken using a fresh sump-full of this grade.  (The dyno was set up for EEC horsepower, i.e. pessimistic.)

Turbolene GTI 15W/50

Max power       127.9 BHP @9750 rpm

Torque             75.8 ft-lbs @ 7300 rpm

After a flush-out and fill-up with Pro S 5W/40 the reading were;

Pro S 5W/40

Max power       131.6 BHP @ 9750 rpm

Torque             77.7 ft-lbs @ 7400 rpm

Then we tried a new experimental grade, Pro R 0W/20, yes, 0W/20.  This wasn’t as risky as you may think, because this grade had already done a season’s racing with the Kawasaki World Superbike team, giving them some useful extra power with no reliability problems.  (But it must be said, they were only interested in 200 frantic miles before the engines went back to Japan.)

Pro R 0W/20

Max power       134.4 BHP @ 9750 rpm

Torque             78.9 ft-lbs @ 7400 rpm

In other words, 3.7 BHP/2.9% increase from GTI to Pro S, a 2.8 BHP / 2.1% increase from Pro S to Pro R, or 6.5 BHP / 5% overall.  Not bad, just for changing the oil, eh?  More to the point, a keen bike owner would have paid at least £1000 to see less improvement than this using the conventional approach of exhaust / intake mods, ignition re-mapping etc.

Am I recommending 0W/20 for high performance engines?  Well, perhaps not!  The Pro S 5W/40, which is a ‘proper’ PAO/ESTER shear-stable synthetic, will look after a powerful engine better than a heavier viscosity ‘cave at the back’ conventional oil, and provide a useful few extra BHP.  (On the other hand, the 0W/20 was very thoroughly developed to give good anti-wear protection.  I think I was on ‘Blend 6’ before Kawasaki was happy with it!) 

 The End

(as it says in all the best bedtime books)

However, as with all good things in life, we don’t live a perfect world of perfect motorcars and therefore we have to look at the lubrication trade-off between longevity, reliability, power, and cost, relative to the vehicle in which the oil is being used (a scruffy banger Super Gti  with 193,000 miles on the clock is a very different proposition to your spanking new Lancer).   Which is why many car manufacturers  (and probably your local dealer) recommend a 5W/40 (such as Pro S 5W/40); but only the most committed competitor would want, or need, the 0W/20 Pro R for the extra 5% power.

So… perhaps there is a Hill-climber, sprinter or rally type out there who is willing to take a punt on the 0W/20 in the never-ending quest for power?  Could be interesting.

JR

John Rowland

R & D Automotive Chemist at Fuchs Lubricants (UK) plc, who make Silkolene oils.

(Under pressure, admits to falling off anything on 2 wheels, so drives a Morgan 3-wheeler instead, or a 42 year-old Mk 1 Sprite for luxury travel, with the decadent refinements of doors, synchromesh, and hydraulic brakes.)

NOTE:

Since this article was originally written many competitors in racing, from Formula Ford through to F3 and beyond have started to successfully use Pro R 0w/20.

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