What is Viscosity?
by definition, is an oil’s resistance to flow and shear. Water is a low
viscosity fluid; syrup is a high viscosity fluid. Oil, like syrup, is a
Newtonian fluid that, as you increase the temperature, the viscosity lowers,
meaning it flows faster, or more easily.
What is Viscosity Index (VI)?
It is the
rate of change of viscosity between two temperatures. The lower the VI, the
more the drop in viscosity as the oil warms up. The higher the VI value,
the less the drop in viscosity as the oil warms up.
this is modified by the inclusion of additives to form multi-grade viscosity
oils. Synthetics usually have a higher viscosity index (less viscosity
change against temperature) than mineral oils.
What is the
importance of viscosity in my engine?
viscosity will determine how easily the oil is pumped to the working
components, how easily it will pass through the filter, and how quickly it
will drain back to the engine. The lower the viscosity the easier all this
will happen. That is why cold starts are so critical to an engine because
the oil is cold, so relatively thick.
lower the viscosity, the less the load the oil can support at the bearing on
the crankshaft. The higher the viscosity, the better the load support.
Even this, however, has a trade-off, since the higher the viscosity, the
more the drag at the bearing, and hence, potential power loss. So a
compromise is chosen to minimise power loss, but maximise load support.
Stick to the recommended viscosity for your engine.
domestic use, engine life is important. For motorsport, engine life is not
critical, winning is, so these high performance engines use lower viscosity
oils to maximise power output to the wheels.
How is viscosity rated on
an engine oil?
Society of Automotive Engineers (SAE) developed a scale for both engine and
transmission oils. The measurement is undertaken in a laboratory in
accordance with the American Standards and Tests Methods (ASTM) and is
usually rated from 0 to as high as 60 for engine oils and from 75 to 100 for
gear oils. A 40 engine oil has a similar viscosity to a 90 gear oil.
steps are 0W, 5W, 10W, 15W, 20W, 30, 40, 50 60 for engines and 75W, 80, 90
and 100 for transmissions.
What is a multi-grade oil?
early 60’s, most oils were a mono-grade, typically either a 30, 40 or 50
grade viscosity. The higher the ambient temperature, the higher the number
used. This did mean that a cold start could be quite damaging for an
engine, so in Winter, the oil would have to be drained and replaced with a
lower viscosity oil.
Multi-grade oils were developed to allow year round use. They are typified
by a two number system with a W. For example, 10W30. The first number
refers to the viscosity of the base oil used, and W is generally understood
to mean Winter (although some suggest W for Weight of the base stock). To
achieve a useful viscosity at operating or warm temperatures, the 10W has
additives (also known as Viscosity Index Improvers), which are like tightly
wound coils that unwind as the oil warms up. The expanding coil causes the
viscosity to increase and perform like a thicker oil. However, over time,
these additives become susceptible to shear and lose some of their
Historically, with older engines having looser clearances, and especially in
the Mini/1100’s with the gearbox in the sump, these engines still generally
require a thicker oil such as a 20W50 or 15W40, unlike modern cars which run
on 10W30 or even 0W30.
How are oils
regulated or compared in the industry?
American Petroleum Institute (API) categorises the formulation of oils
according to a standard. Typical engines of the period are used as a test
bed, and a number of tests are run to ascertain the performance of an oil
under test. If it meets or exceeds the parameters of the test then it can
be classified in that category. However, oils in the same category do not
necessarily have equal performance, they may only meet, not exceed, the
requirements of that category. Current ratings are based around standard
engine design of the moment, so modern oils are designed to withstand higher
power outputs, with multi-valve and overhead cam etc reaching higher rpm
What are these categories?
or Spark Ignition engines, there is a ‘S’ rating, and for diesel or
Compression Ignition engines, there is a ‘C’ rating.
Spark Ignition Oils:
ratings started in the 50’s for typical engines with an API SA rating. By
the 70’s it had moved onto an API SC rating. During the Nineties it rapidly
moved up to as high API SL. The oil required will depend on your driving
needs, i.e. daily commuting versus track-day or motorsport stuff. For the
latter, the best you can afford, but for commuting, an API SH or API SJ
should be more than adequate. For older engine designs, avoid the very
latest API spec, use one that is designed for that design of engine, such as
an API SF.
For Compression Ignition Oils:
classification is currently at API CI-4, although this is designed for
exhaust gas recirculating (EGR) engines where the soot loading is
consequently higher as a result of remixing the emissions into the air
intake. Again, the rating has moved on rapidly during the 90’s, and the
same comment applies for spark ignition, that is, buy the best you can
afford. For fleet use in off-highway equipment, the way to extend the oil
drain interval is to go for this API CI-4 oil.
Where can I find the API rating?
rating is usually shown in a doughnut or circle on the side of the oilcan,
or may be listed in the small print. Not all manufacturers show this on
their containers, possibly for the reason that not many people know what it
means, or more cynically, because it hasn’t been fully tested!
Is a major
brand better than a smaller, unknown brand?
taken in by marketing; check that an oil is a fully synthetic as some claim
to be ‘synthetically engineered’ whilst only being a semi-synthetic.
independent testing that I have been involved with in the past, there have
been significant differences in wear control between major brands, and don’t
write off smaller brands, either. Some smaller brands specialise in a type
of oil and do not carry the overheads of a major supplier. They dedicate
their facilities to a type of base stock, operate in a cleaner manner, and
use higher quality base stocks with specifically formulated additive
packages. But it will cost more.
example is Royal Purple, used predominantly by the motorsport fraternity,
with good reason. Actual evidence has shown their oil to cope with the
stresses imposed on engines such as the Subaru Impreza and Smart where major
brand oils have failed.
What About Brake Fluids?
Dot 4 and DOT5.1 is ester
based and is hygroscopic, that is, it absorbs moisture from atmosphere.
Along with high temperature abuse, the moisture reduces performance of fluid
and lowers boiling point from around 450°C to about 150°C.
So..............run a track day or pull a caravan and you work the brakes
hard, thus fluid starts boiling fairly quickly! Boiling means bubbles which
means compressibility and spongy or zero power transfer to the slave
cylinders. Water absorption (through the rubber hoses and seals) also leads
to rust in the slave cylinders and consequent pitting and leakage past the
seals. So change the fluid every two years or sooner as per the recommended
Dot 5 is silicone based, or also known as synthetic, and although more
viscous leading to slightly less responsive brakes, it doesn't eat paint
work, doesn't absorb moisture and can run to extreme temperatures, and only
needs replacing every 3 years as opposed to every year with Dot4.
And if you open a Dot 4 bottle, notice it has a foil seal. Use what you
need, then chuck the rest away, because on the shelf, in the average garage,
without the foil seal, it will be useless within 30 days having absorbed so
And if you
want maximum transfer of power to the wheel cylinder, consider braided
hoses, which minimise bulging and thus a loss of power transfer.
And as for Shell Optimax fuel!
offers the following because of a better burn capability and a higher RON
number 98, compared to other super unleaded fuels of RON 97:
Better power output (probably 5% max)
Better, cleaner emissions from the exhaust
Maintains a cleaner fuel system (pipes and
injectors, as well as in the combustion chamber minimising depositing)
have a knock sensor that can adjust the engine management/timing, then you
will see all 4 benefits. Unfortunately, the standard K series does not so
most people don't notice the improvements in the short term. The KV6 does
benefit so I use it regularly. Still on the 4 points above, it is difficult
to justify the extra costs.
On the K series, I use it in the wife's 214 in the hope that at least points
3 and 4 are a benefit. She does a lower mileage than I do, so it doesn't
cost as much for this 'insurance' policy and doing my bit for the
Don't expect immediate benefits from it, you need to use it regularly to see
the advantages. If your car has suffered from poorer quality fuels over a
year or more, then several tanks will help you recover lost efficiency
through clean up, not increase performance per se, unless you have the
engine management system to cope.
You could talk to your garage about adjusting the timing to benefit from it
if you use it regularly, especially on an older car.
plan to keep the car long term, then invest in a good quality fuel and oil
and stick with them as the reduces long term problems. If you plan to sell
in next couple of years, then don't bother too much.
seeking ultimate power, then an additive in the fuel can help raise the RON
to as high as 100 with Optimax, but you will need to ensure the timing is
set up for this gain.