Oil Labelling Explained

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What’s written on your oil bottle and what does it mean?

This post may seem like going back to basics but I am constantly surprised by the amount of people who do not know or understand what is written on a bottle of oil and therefore no idea of what they are buying/using.


  1. The purpose for which it is intended (i.e. Motor oil, Gear oil etc)
  2. The viscosity (i.e. 10w-40, 5w-30 etc for Motor oils and 80w-90, 75w-90 etc for Gear oils)
  3. The specifications that it meets (should contain both API and ACEA ratings)
  4. The OEM Approvals that it carries and the codes (i.e. MB229.3, VW503.00, BMW LL01 etc)

Ignore the marketing blurb on the label it is in many cases meaningless and I will explain later what statements you should treat this with some scepticism


The Basics




Most oils on the shelves today are Multigrades, which simply means that the oil falls into 2 viscosity grades (i.e. 10w-40 etc)


In a 10w-40 for example the 10w bit (W = winter, not weight or watt or anything else for that matter) simply means that the oil must have a certain maximum viscosity/flow at low temperature. The lower the “W” number the better the oils cold temperature/cold start performance.


Viscosity link on Wikipedia


Specifications are important as these indicate the performance of the oil and whether they have met or passed the latest tests or whether the formulation is effectively obsolete or out of date. There are two specifications that you should look for on any oil bottle and these are API (American Petroleum Institute]) and ACEA (Association des Constructeurs Europeens d’Automobiles) all good oils should contain both of these and an understanding of what they mean is important.





SG Introduced 1989 has much more active dispersant to combat black sludge
SL Introduced 2001, all new engine tests reflective of modern engine designs meeting current emissions standards










A1 Fuel economy petrol
A2 Standard performance level (now obsolete)
A3 High performance and/or extended drain
A4 Reserved for future use in certain direct injection engines
A5 Combines A1 fuel economy with A3 performance
B1 Fuel economy diesel
B2 Standard performance level (now obsolete)
B3 High performance and/or extended drain
B4 For direct injection car diesel engines
B5 Combines B1 fuel economy with B3/B4 performance

Note: SAPS = Sulphated Ash, Phosphorous and Sulphur.




Oil Companies send their oils to OEM’s for approval however some older specs are easily achieved and can be done so with the cheapest of mineral oils. Newer specifications are always more up to date and better quality/performance than the older ones.


VW – 500.00, 501.00 and 505.00


MB – 229.1


BMW – LL98





  • Recommended for use where.........
  • May be used where the following specifications apply..........
  • Approved by..........(but with no qualification)
  • Recommended/Approved by (some famous person, these endorsements are paid for)
  • Racing/Track formula (but with no supporting evidence)

Also be wary of statements like “synthetic blend” if you are looking for a fully synthetic oil as this will merely be a semi-synthetic.


Why oils lose viscosity with use

Viscosity Index Improvers. An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.

VI Improvers are heat sensitive long chain, high molecular weight polymers that minimise the viscosity loss of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.

"Shearing" The long chain molecules in VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing viscosity. This "shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules. The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.

This shearing not only reduces the viscosity of the oil but can cause piston ring sticking (due to deposits), increased oil consumption and increased engine wear.

Like basestock quality, VI Improvers also vary in quality. As with many items the more you pay, the better the finished article and more expensive, usually synthetic oils are likely to incorporate better VI improvers. All other things being equal the less VI improver an oil contains, the better it will stay in grade by resisting viscosity loss.

Which oils require more VI Improvers? There are two scenarios where large amounts of these polymers are required as a rule.

Firstly in "wide viscosity span" multigrades. By this I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 (diff 45) and 10w-60 (diff 50) are what is termed as "wide viscosity span" oils.

Narrow viscosity oils like 0w-30 (diff 30) or 5w-40 (diff 35) require far less VI Improvers and therefore are less prone to "shearing".

Secondly, mineral and hydrocracked (petroleum synthetic oils) require more VI Improvers than proper PAO/Ester (Group IV or V) synthetic oils as they have a higher inherent VI to begin with, this is due to differences in the molecular structure of the synthetic base oils compared to mineral oils.

It is a fact that many synthetics require significantly less VI Improver to work as a multigrade and are therefore less prone to viscosty loss by shearing.

HTHS - High temperature High shear A relatively new oil test/specification, the oil is subjected to high temperature 150C and is mechanically sheared at 1 x 10^6 per second. A value of 2.8 or is considered the minumum for normal bearing wear. Here are some HTHS numbers on well known oils, the higher the number the more shear stable the oil; 4's and 5's are excellent numbers and demonstrate basestock quality e.g. Silkolene and Motul.

Mobil 1 0W40 HTHS 3.6

Castrol RS 10w-60 HTHS 3.7

Castrol RS 0w-40 HTHS 3.7

Silkolene PRO S 5w-40 HTHS 4.07

Motul 300V 10w-40 HTHS 4.19

Motul 300V 5w-40 HTHS 4.51

Redline 5w40 HTHS 4.6

Redline 10W40 HTHS 4.7

Mobil 1 15w-50 HTHS 5.11

Silkolene PRO S 10w-50 HTHS 5.11

Silkolene PRO R 15w-50 HTHS 5.23

Motul 300V 15w-50 HTHS 5.33

Redline 15W50 HTHS 5.8

ROAD: Most semi synthetics are past their best between 3-5000 miles, synthetics are good for 9000-12000 miles on the road.

TRACK: Change periods will be determined by quality of the oil i.e. how shear stable it is under track conditions. A decent race oil will take about 10 hours of hard track use and a semi-syntehtic or hydrocracked base stock will last about an hour before the oil begins to lose viscosity.

Are 5w and 0w oils too thin?

I read on many forums about 0w and 5w oils being too thin.

0w-40, 5w-40, 10w-40 and 15w-40 are all the same thickness (14 centistokes) at 100degC.


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?


Now, ALL oils are thicker when cold. Confused? It's true and here is a table to illustrate this.

SAE 40 (straight 40)

Temp degC Viscosity (thickness)
0 2579cst
20 473cst
40 135cst
60 52.2cst
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 an 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 an 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.

Other sources of information

Opie Oil

Wikipedia on Motor Oil

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