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Many motorists take the view that one oil is much the same as another, so why spend more than the minimum when purchasing? Well, although oils may look and feel very similar, their specifications are as wide and varied as the number of brands on the market. The bottom line is that filling your engine with a poor quality or incorrect specification of oil will drastically reduce its service life, in some cases within just a few miles! The British Lubricants Federation issues a guidance leaflet with this clear message:

"Choosing the correct engine oil can save motorists the price of many gallons of fuel a year in addition to prolonging service life"

So what is so special about oil? To understand this you must first recognise that lubricating oil, apart from reducing wear and friction, provides four additional and very important functions:

1) it acts as a cooling medium;

2) it keeps the internal components clean;

3) it prevents corrosion;

4) it reduces noise.

Modern multi-grade oils are no longer a base product refined from crude oil. They are also a complex alchemy of additives such as 'viscosity index improvers', 'pour point depressants', 'detergent dispersants', and many others besides. Add to this all the various synthetic oils now being produced and you begin to realise that these oils are not all the same.

So how do you tell the difference? Oil specification is determined by two criteria namely Viscosity (or thickness) and Performance (or quality). We shall deal with these separately:

Viscosity is simply the 'thickness' of oil. Most engine wear occurs during the critical moments following a cold start. High viscosity (thicker) oils circulate slower than low viscosity oils and the colder the oil, the thicker it will be. The Society of Automotive Engineers (SAE) classification system establishes the viscosity characteristics for the lubricating oil industry. All multi-grade oils are tested for two criteria i.e. maximum viscosity when cold (at temperatures as low as -40°F/-40°C) and secondly, minimum viscosity when hot (at 212°F /100°C). The first test is referred to as a 'Winter' test, hence the 'W suffix which denotes the thickness (20W) or thinness (5W) of an oil during cold start conditions. The second test is designed to give a clear indication of the oil's minimum viscosity during normal operating temperatures. As with the winter test, the higher the number the higher the thickness of the oil. To avoid confusion it must be noted that the methodology used for these two tests is entirely separate and bear no relation to each other. All oils get thinner (lose their viscosity) as they get hotter so don't be fooled into believing that say a 20W/50 oil gets thicker as it gets hotter - it doesn't! Fig. 1 illustrates the ambient temperatures all current multi-grade oils will operate within. But this is only part of the selection process since, for most of the UK, any of those nine grades could be used. The type and age of an engine will also dictate which grades can or cannot be used and the manufacturers recommendations are paramount Modern engines are machined to very close tolerances enabling them to use the extremely thin oils developed by the oil industry to improve fuel consumption and emissions. Using a thicker than specified oil in these engines will not only affect fuel consumption and emissions. Engines will be more difficult to start, hydraulic followers will over pressurise preventing the valves closing completely, and during cold start the oil will not reach all parts of the engine quickly enough, thus initiating premature wear! Conversely, older engines with their larger operating clearances demand the higher viscosity oils to achieve an adequate oil film thickness. Using a modern low viscosity oil in engines such as a X/flow or V6 Essex could be a recipe for disaster with a danger that the oil film will break down in the bearings, allowing metal to metal contact to take place with the inevitable consequences!

Performance standards are currently laid down for three automotive engine groups namely: petrol light duty diesel, and heavy-duty Diesel These standards are currently controlled by three main governing bodies i.e. the American Petroleum Institute (API); the Association des Constructeurs Europeans de ('Automobiles (ACEA); and the International Lubricant Standardization & Approval Committee (ILSAQ). In addition to these organisations, individual vehicle manufacturers are increasingly issuing their own specifications (e.g. Ford GM, VW, etc). Since the API & ACEA standards are universally quoted on oil packaging we shall discuss just these two in relation to petrol engines only. The API 'SL' classification is the current benchmark for top quality petrol engine oils and all new engine warranties require a minimum 'SH' specification of oil to be used.

 Figure 2 illustrates the improvements made to oil quality since the original 'SA' standard was introduced and how significantly oil performance has advanced over the last 20 years. In 1996 a new European standard was introduced (ACEA) to improve the clarity of test programmes and assert minimum quality standards from the oil blenders. Below are listed the three specifications for petrol engines (A1;A2;A3) which are then suffixed by the year the standard was changed.


Current Standards:

Regardless of the age of an engine, any newer specification can be used providing the viscosity is correct. However, when you choose to use a higher specification oil the oil change period must never be extended. Nearly all the oil sold will have me API and/or ACEA classification printed on the container and, unless it is a very old vehicle, we would not advise using any oil below API 'SF quality (all ACEA standards meet or exceed API 'SF'). If there is no API or ACEA classification on the container - leave it where you found it!

Synthetic Oils

Although commanding a premium price these do offer excellent value for money through 'state of the art' technology. Synthetic oils offer advantages in most engines, particularly in respect of performance under extreme service conditions such as are encountered in turbo-chargers. Operating within such severe environments the high thermal stability and resistance to degradation of synthetic oils resists the formation of carbon deposits which not only affect performance but may ultimately cause an expensive turbo failure. In comparison to mineral based oils, synthetic oils not only offer advantages under high temperature conditions but also at low temperatures where their very good cold flow properties allow the latest 0W or 5W fuel efficient oils to be produced without encountering excessive oil consumption. Additional benefits of synthetic oil are:

Purchasing synthetic oil however, is not always a simple procedure since there are several differing interpretations of the word synthetic and also the availability of 'semi' or 'partially' synthetic oils. These semi or partially synthetic oils are generally a mixture of conventional mineral oils with an undefined proportion of synthetic oils which can offer some of the advantages of 100% synthetics but never the full technical advantages Whilst partial synthetics are excellent oils at a lower cost, if you want the very best engine protection your money can buy then insist on Fully or 100% synthetic - and check the label!

Some final tips on oil changing:


Frank Sherlock - Valvoline Oil Company The British Lubricants Federation

Graham Lord - Millers Oils Ltd

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Last updated 1/1/17
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