Information on Oils
Technical oil discussion by Oilman – Part 1
Submitted by: Oilman
Disclaimer
This article is intended as helpful information. If you are in any doubt, consult a qualified individual. The author or the GT4 Drivers Club can not be held responsible for any loss or damage resulting from use of this article.
Oil Discussion Part 1 – Common Misconceptions
I read on many forums about 0w and 5w
oils being too thin. I will try to explain it without getting over technical
and we'll go from there.
0w-40, 5w-40, 10w-40 and 15w-40 are all the same
thickness (14cst) at 100degC.
Centistokes (cst) is the measure of a fluid's resistance
to flow (viscosity). It is calculated in terms of the time required for a
standard quantity of fluid at a certain temperature to flow through a standard
orifice. The higher the value, the more viscous the fluid.
As viscosity varies with temperature, the value is
meaningless unless accompanied by the temperature at which it is measured. In
the case of oils, viscosity is generally reported in centistokes (cst) and
usually measured at 40degC and 100degC.
So, all oils that end in 40 (sae 40) are around 14cst
thickness at 100degC.
This applies to all oils that end in the same number, all
oils that end in 50 (sae 50) are around 18.5cst at 100degC and all oils that
end in 60 (sae 60) are around 24cst at 100degC.
With me so far?
Great!
Now, ALL oils are thicker when cold. Confused? It's true
and here is a table to illustrate this.
SAE 40 (straight 40)
Temp deg C.........................Viscosity (thickness)
0..........................................2579cst
20..........................................473cst
40..........................................135cst
60..........................................52.2cs t
100........................................ 14cst
120.........................................8.8cst
As you will see, there is plenty of viscosity at 0degC,
in fact many times more than at 100degC and this is the problem especially in
cold weather, can the oil flow quick enough to protect vital engine parts at
start up. Not really!
So, given that a sae 40 is 14cst at 100degC which is
adequate viscosity to protect the engine, and much thicker when cold, how can a
0w oil be too thin?
Well, it can't is the truth.
The clever part (thanks to synthetics) is that thin base
oils can be used so that start up viscosity (on say a 5w-40 at 0degC) is
reduced to around 800cst and this obviously gives much better flow than a
monograde sae 40 (2579cst as quoted above).
So, how does this happen, well as explained at the
beginning, it's all about temperature, yes a thin base oil is still thicker
when cold than at 100degC but the clever stuff (due to synthetics again) is
that the chemists are able to build these oils out of molecules that do not
thin to less than 14cst at 100degC!
What are the parameters for our recommendations?
Well, we always talk about good cold start protection, by
this we mean flow so a 5w will flow better than a 10w and so on. This is why we
recommend 5w or 10w as the thickest you want to use except in exceptional
circumstances. Flow is critical to protect the engine from wear!
We also talk about oil temps, mods and what the car is
used for. This is related to the second number xw-(XX) as there may be issues
with oil temperatures causing the oil to be too thin and therefore the
possibility of metal to metal contact.
This is difficult to explain but, if for example your oil
temp does not exceed 120degC at any time then a good "shear stable"
sae 40 is perfectly capable of giving protection.
"Shear stability" is important here because if
the oil shears it thins and that's not good!
However, if you are seeing temperatures in excess of
120degC due to mods and track use etc then there is a strong argument to using
a sae 50 as it will have more viscosity at these excessive temperatures.
There are trade offs here. Thicker oils cause more
friction and therefore more heat and they waste power and affect fuel
consumption so it's always best to use the thinnest oil (i.e. second number)
that you can get away with and still maintain oil pressure.
There is more but this post is too long already so lets
keep it to basics.
Cheers
Simon
Here are the figures for viscosity in cst (centistokes) and
temperature for different oils.
The first numbers are sae numbers for straight 30, 40, 50 and 60. The second
set of numbers is for various multigrades as they are obviously thinner when
cold than straight oils due to lighter viscosity basestocks being used.
Monogrades
DegC...........0.....20......40......60......100......120
Sae 30....1600....315.....95.......39......11........7...
Sae 40....2579....473....135.......52......14.......9....
Sae 50....4592....771....205.......75......18.......11..
Sae 60....7865...1218...303.......105.....24.......14..
Multigrades
DegC...............0...............10............40..............100
0W/20............329............181...........46...............9..
5W/40............811............421...........92..............14..
10W/50..........1039...........539..........117.............18..
15W/50..........1376...........675..........130.............18..
20W/50..........2305..........1015..........148.............18..
These two graphs demonstrate the fact that a monograde is the same thickness at
100degC as a multigrade of the same sae number but the distinct benefits that a
multigrade brings at lower temperatures. This is obviously benefits cold crank
wear as the rate of flow of multigrades is much better at lower temps.
There is no temperature where oil suddenly starts to flow better. A 10W/40 for
example will flow between -25C and 100C or more, but there is a big difference
in the rate of flow (True viscosity at -25C is about 7000 Centistoke(cSt)
units, dropping to 14 at 100C. ‘Viscosity’ is just another name for ‘rate of
flow’.
The question is, at what (sump) temperature is the oil at a viscosity that
suits a modern high-RPM engine.
Present day designs seem happy on an oil viscosity of 10 to 15 cSt. (But many
are OK on less than 10.) 30cSt is too high at high RPM. It can lead to foaming,
air entrainment and cavitation.
............Temp. for 30cSt (Deg. C).......Temp. for 15cSt.......Temp. for
10cSt
5W/40..................71...........................90........................117...............
10W/40................70...........................99........................118...............
10W/50................80...........................109.......................130..............
10W/60................89...........................119.......................142..............
This shows that a 5W/40 or a 10W/40 is perfectly adequate for all engines
except those that run unusually high temperatures.
Also, thick oil can lead to trouble unless properly warmed up before high RPM
is used.
I hope that these figures at least give some insight to what actually happens
to the oil inside your engine at different temperatures.
Cheers
Simon
