This was from Oilman in SN. Worth a read.
"Ok, I'm fed up with all the rubbish that is posted when anyone asks a question on oil so, here is some proper advice written by an expert.
This advice is copied but the Author is well known to me and I have his permission to post it.
If you wish to understand what it's all about then you'll not find a better written article about "Lubricating the Subaru"
Lubricating the Subaru.
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 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 as well.
The trouble is, all this is just as true for Henry Fords original Model T engine as it is for the Subaru or any other high output motor. So where is the difference?
The Model T, with 10bhp/litre at 2,000rpm and a single underhead camshaft, was filled with a thick, greenish liquid from somewhere near the bottom of the distillation colums on the Pennsylvania oilfields. It did a vague tour of the internals by guesswork (there was no oil pump) at a temperature around 50 degC, and lasted for 1,000 miles. On the plus side, some of the impurities acted as anti-wear and detergent chemicals. They didnt 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 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 its way along narrow oil ways to the valve gear. The turbo bearing 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 the problem with modern oils only 15 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 degC without evaporating or carbonising, and maintain oil pressure.
Unmodified thin oils simply cant 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 multigrade.
But, this is all too basic! What I have just written was and is relevant to a 1958 Morris Minor.
The questions that Subaru owners need to ask are: Will this thin 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 21st century questions which cannot be answered by a basic 1990s 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 oil.
Oil viscosities are accurately measured in units called Centistokes at exactly 100 degC. These fall into five high temperature SAE catagories:-
SAE No.........20..........30...............40........ .........50...............60
VisRange..5.6-<9.3..9.3-<12.5...12.5 <16.3....16.3-<21.9....21.9 - <26
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 its 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 (it doesnt mean weight). These simulate cold start at different non-ferrous monkey endangering temperatures from 15 degC for the 20w test to a desperate 35 degC for 0w.
For example, an SAE 5w-40 oil is one that has a viscosity of less than 6600 units at 30 degC, and a viscosity of about 14 units at 100 degC.
Now, those of you 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 30 degC, and 14cst is plenty of viscosity when the engine is running normally. (A lot more could damage the engine.
The vital point is, a monograde 40 would be just like candle wax at 30 degC, and not much better at 10 degC. It would even give the starter motor a fairly difficult time at 0 degC. (At 0 degC, a 5w-40 has a viscosity of 800 but the monograde 40 is up at 3200!)
Another basic point about wide ranging multigrades such as 5w-40 or 0w-40 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 its ready-made and cheap. This is the estate agents 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 and 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 130 degC 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 as 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 nurky 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-mans 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 hydrocracked 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 oils). Looked at non-basically there are three broad catagories, each containing dozens of types and viscosity grades:-
