Difflock Guide to Oils and Lubricants

Part 1 - Where it all starts
Part 2 - What it all means
Part 3 - Selecting the best oil for your engine.

In the last feature, we discussed the various types of engine oil and the tasks they have to perform. We also discovered that buying the most expensive oil available can actually be bad for an engine if it’s the wrong type and viscosity. In his feature we will look in more detail at the factors affecting engine oil selection, and how to make the best choice for your vehicle.

Viscosity.

Choosing the right viscosity oil for your vehicle is the first and probably the most critically important decisions you can make. Viscosity is the measure of how thick or thin the oil is; by way of example, water flows very freely and so has a low viscosity but cold treacle flows very slowly and so has a thick viscosity. It doesn’t matter how wonderful the is the quality of the oil, nor how many special additives it contains, if you get the viscosity wrong you will:

Reduce performance
Reduce economy
Increase oil consumption
Accelerate engine wear

So why is viscosity so important?.

Well, if you choose an oil that is too thin it will:

Be squeezed out of high pressure contact areas allowing metal to metal contact and wear.
Leak past the rotors of the oil pump reducing both oil pressure and flow
Leak past oils seals, bearings and running surfaces reducing both oil pressure and flow
Leak past running surfaces such as the pistons and bores increasing oil consumption and reducing economy and performance

If you choose an oil that is too thick it will:

Increase frictional losses, sapping economy and performance
Fail to flow quickly enough to disperse heat from critically stressed areas of the engine.
Fail to circulate quickly enough to lubricate vital areas of the upper engine during cold starting
Increase stress and strain on the starter motor and battery during cold starting.

So how do you determine an oils viscosity?

Luckily for us, lubricant companies use a global standard to tell you the viscosity of their oils. The standard used is the SAE (Society of Automotive Engineers) Viscosity Grade. However, you will recall from our last article that an oil’s viscosity can also change with its temperature. So the SAE standard measures the viscosity of oils at specific temperatures, depending on whether it is a Monograde or Multigrade

Monograde Oils

These have their viscosity measured at around 100 degrees C only and a single number is quoted

Typical examples of Grades are SAE30, SAE40 and SAE50.

The lower the number, the thinner the oil at any given temperature

Multgrade Oils

Multigrade oils remember, have Viscosity Improvers added to limit the oil’s tendency to thin as it gets hotter. Thus Multigrade Oils are measured at two different temperatures to tell you how the oil behaves when both hot and cold. The hot measurent is again performed at around 100 degrees C but the cold measurement is taken at around minus 18 degrees C

Examples of Grades are SAE5W30, SAE10W40 and SAE15W50.

Again the lower each number, the thinner is the oil at any given temperature.

Lets take the 10W40 grade and explain this in a little more detail.

The first number, 10, refers to the oils viscosity at low temperature (minus 18 degrees C) It means that at minus 18 degrees C, the oil will have the same viscosity as would an SAE10 Monograde at that temperature. However, as the oils heats up those Viscosity Improvers get to work, preventing the Multigrade oil thinning out like an SAE10 Monograde and so at 100 degrees C, the second number 40 tells us that the Multigrade actually now has the same viscosity as would an SAE40 Monograde at that temperature.

You may have noticed the ‘W’ in the middle of the Multigrade numbers. This means that at low temperatures, the oil has not only been measured for its viscosity, but also for its pumping characteristics to both reach the oil pump and sustain acceptable oil pressure during low temperature engine cranking (ie when cold starting on freezing winter mornings). Hence the ‘W’ specifically means the oil is suitable for low temperature (Winter) use. Grades without the ‘W’ rating are intended for use at higher temperatures and so must NOT be used where repeated, very cold starting is required (ie during those long cold winter months).

It can be seen that Multigrades give better all year round performance than Monogrades where seasonal temperature shifts from below freezing to the heat of summer. Equally, it is important that the oil has a ‘W’ grading for cold winter use.

However, one drawback to Multigrades is that over time, the Viscosity Improvers shear down within the engine and so are no longer able to control the oil’s tendency to thin as it gets hot. This then is another good reason for changing Oils strictly in accordance with the lubricant manufacturer’s and vehicle manufacturer’s recommendations (taken together).

Now we understand what a Viscosity Grade means, we can start to think about why different engines require different viscosities.

If we take the case of a modern designed and manufactured engine, its materials, components (oil pump) and running tolerances (including changes due to thermal expansion) will all usually be specified to permit the continual use of lower viscosity oils, especially Synthetics. Setting aside the fact the Synthetics reduce wear (improving reliability) and last longer than mineral oils, modern vehicle manufacturers favour Synthetics because of the fuel economy benefits which they can market to potential customers. Certain emissions can also be reduced by using Synthetics.

However, older designs of engine, especially those with appreciable wear, are not designed for running on low viscosity oils and indeed the use of such oils will give rise to the problems already outlined.

It should also be remembered, that as an engine wears, it may be prudent to switch to a slightly thicker viscosity of oil, since this will help maintain oil pressure and reduce oil consumption and visible smoke.

Let’s not forget ambient temperatures too. For all year round UK use, the prevailing ambient temperature will nearly always fall between –10 &Mac176;C to +30 &Mac176;C. However, if you live near the Arctic Circle the prevailing temperatures are likely to be much, much lower. By the same token, in the Tropics, those temperatures will be higher. As we’ve already discussed, these temperatures will change the viscosity of the oil in the engine (especially from start up) and therefore vehicle manufacturers will recommend thinner, lower viscosity multi-grades for colder climates and thicker, higher viscosity multi-grades for hotter climates. Hence, if you’re taking your vehicle from Pole to Pole, you’ll need a relatively thin oil at the Poles, changing to a thicker oil as you move progressively nearer to the hot Equator. Luckily (or perhaps unluckily), most of us seldom stray so far in our 4x4 as to move from one climate zone to another. But as can be seen from the following chart (taken from a Discovery 200 TD engine Workshop Manual), you should use the prevailing winter and summer temperature range where you live to help determine the best viscosity for your engine.

As can be seen from the chart, if you intend driving your vehicle in predominantly sub-zero temperatures, a 5W40 viscosity grade would be ideal. However, for a temperate climate such as the UK, a thicker 10W40 grade of oil would be better suited and is likely to be more readily available and more cost-effective than a 5W40. Equally, the slightly thicker 10W40 grade will be more tolerant of engine wear in temperate climes. However, as can be seen, if your planning a trip across the Sahara then a 10W50, or even 15W50, grade will be required to cope with those very much higher daytime temperatures.

Looking at the chart, you might think that something like a 5W50 grade is the best solution of all since it appears to cover all temperature ranges. In reality however, such a grade is likely to be a little too thin for anything less than sub zero cold starting and a little too thick for anything less than sustained running at higher engine temperatures. You should also bear in mind that an oil with so great a Viscosity span (from 5 to 50) will not be as long lived as other oils since the Viscosity Improvers shear down more easily than those in oils with lesser Viscosity Spans. Hence the key is to find an oil with the smallest Viscosity span that most closely matches the low winter and high summer temperatures in your location.

Summarising all of the above, it should be clear now that the very first thing above all else that you should look for when choosing an oil, is that its viscosity grade is correct for your engine. You should first refer to your manufacturer’s handbook to determine the best climate-matched viscosity span for your location and then you should make a small allowance for any known engine wear or oil consumption, selecting a slightly thicker grade if such wear is significant.

Mineral, Semi or Fully Synthetic?

As we mentioned in the last feature, modern engines are designed and manufactured to use lower viscosity oils than their forefathers since this gives better economy and performance. We also mentioned that today’s faster roads and longer journeys at higher road speeds have driven the development of Synthetic Oils.

As a general rule, Synthetic base stocks tend only to be used in lower viscosity oils since it is in these thin oils that the higher film strength (the oil’s resistance to be squeezed out of high pressure contact areas) of synthetic oils really give greatest advantage. Additonally, because thin oils tend to be used in modern engines, synthetics give greater longevity to the lubricant in the conditions found in such engines.

Broadly speaking, if your engine requires a viscosity grade of SAE 10W40 or lower (thinner) you should look for a semi synthetic or fully synthetic oil for best results. This is especially true if the vehicle is to be used mainly for those higher speed journeys or perhaps, frequent towing.

Viscosities higher (thicker) than SAE10W40 don’t commonly have a synthetic content but don’t worry, engines requiring these thicker grades are generally less thermally and mechanically stressed than those requiring thin grades so good quality plain mineral oils still give excellent protection. So if you are currently using a higher viscosity plain mineral oil in your engine don’t be tempted to swap to a lower viscosity grade just because it has a synthetic content.

Quality

Whilst the SAE Viscosity Grade and Mineral or Synthetic content tells you much about the oil’s physical properties, it tells you absolutely NOTHING about the quality of the oil or its suitability for either petrol or diesel engines. So if you buy your oil simply by looking at the SAE viscosity grade alone, you are ignoring the fact that the engine oil also has to cope with other potential problems such as combustion by products, particulates like soot, oxidation, high pressures and high temperatures.

High quality oils are blended from high quality, high purity base stocks and have special additives to combat these problems. Poor quality oils on the other hand have fewer additives and poorer base stocks within.

The speed at which these factors take effect depends on many, many factors. Amongst these are fuel quality, engine load, environmental factors (such as weather), driving style (very important), average journey duration, material selection and quality used in engine construction, engine design, state of engine tune, engine wear etc.

But how do you tell the quality of the oil? Well luckily we can turn to one of those international standards again which grades an oil according to its quality and its suitability for petrol or diesel engines.

There are several of these quality standards in operation but one of the simplest and easiest to use is that of the American Petroleum Institute or API.

The API system splits oils into two categories, one for Petrol (the ‘S’ grades) and one for Diesel (the ‘C’ grades).

In the case of ‘S’ grade oils, it then further splits these into service grades, these grades cover the various stages of Petrol engine evolution through specific model years, as demands on the oil become greater and greater.

‘S’ grade oils then start at SA (now in fact so old that its not commonly used) and go right up to the very latest SJ grade introduced in 1996.

In the case of ‘C’ grade oils however, these are not split by model year but instead by Diesel engine type and application. By way of example, The CF category was introduced in 1994 for off-road, turbo or non turbo, direct and indirect-injected and other diesel engines including those using fuel with over 0.5% weight sulphur.

The current and previous API Service Categories are listed below. Vehicle owners should refer to their owner’s manuals before consulting these charts. Engine oils are categorized based on their performance characteristics and the type of service for which they are intended: S category oils are suitable for gasoline engines and C category oils for diesel engines. Oils may have more than one performance level.

For automotive gasoline engines, the latest engine oil service category includes the performance properties of each earlier category. If an automotive owner’s manual calls for an API SG or SH oil, an API SJ oil will provide full protection. For diesel engines, the latest category usually - but not always - includes the performance properties of an earlier category.

PETROL Engines
Category	Status	Service
SJ	Current	For all automotive engines presently in use. Introduced in the API Service Symbol in 1996
SH	Obsolete	For model year 1996 and older engines. Valid when preceded by certain C categories
SG	Obsolete	For model year 1993 and older engines
SF	Obsolete	For 1988 and older engines
SE	Obsolete	For 1979 and older engines
SD	Obsolete	For 1971 and older engines
SC	Obsolete	For 1967 and older engines
SB	Obsolete	Use only when specifically recommended by the manufacturer
SA	Obsolete	For older engines; no performance requirement. Use only when specifically recommended by the manufacturer

DIESEL Engines
Category	Status	Service
CH-4	Current	Introduced December 1, 1998. For high-speed, four-stroke engines designed to meet 1998 exhaust emission standards.CH-4 oils are specifically compounded for use with diesel fuels ranging in sulfur content up to 0.5% weight. Can be used in place of CD, CE, CF-4, and CG-4 oils
CG-4	Current	Introduced in 1995. For severe duty, high-speed, four-stroke engines using fuel with less than 0.5% weight sulfur. CG-4 oils are required for engines meeting 1994 emission standards. Can be used in place of CD, CE, and CF-4 oils
CF-4	Current	Introduced in 1990. For high-speed, four-stroke, naturally aspirated and turbocharged engines. Can be used in place of CD and CE oils
CF-2	Current	Introduced in 1994. For severe duty, two-stroke-cycle engines. Can be used in place of CD-II oils
CF	Current	Introduced in 1994. For off-road, indirect-injected and other diesel engines including those using fuel with over 0.5% weight sulfur. Can be used in place of CD oils
CE	Obsolete	Introduced in 1987. For high-speed, four-stroke, naturally aspirated and turbocharged engines. Can be used in place of CC and CD oils
CD-II	Obsolete	Introduced in 1987. For two-strokecycle engines
CD	Obsolete	Introduced in 1955. For certain naturally aspirated and turbocharged engines
CC	Obsolete	For engines introduced in 1961
CB	Obsolete	For moderate duty engines from 1949 to 1960
CA	Obsolete	For light duty engines (1940's and 1950's)

So as can be seen, the API system allows you to rapidly determine whether an oil has the correct formulation and high enough quality to suit your particular engine.

Oil changing & Long Drain Oils

Despite any claims to the contrary you may read ALL engine oils degrade in use and MUST be changed regularly to maintain the condition of your engine and its performance.

These are some of the reasons why oils degrade in use:

The Multigrade Viscosity Improvers shear down and so are used up
The Acid Neutralisers are used up combating combustion by products
The long chain oil molecules break down under the influence of heat and high pressure
The detergents become saturated with particulates such as soot
The lighter base stock fractions evaporate off thickening the oil
The oil oxidizes in the presence of air and heat
The anti wear additives shear down and are used up

It’s a simple and very straightforward rule that the better the quality of the oil, the better it and its special additives will be at resisting these effects.

Poor quality oils however, have their additives rapidly used up especially since such oils contain fewer additives in less concentration.

So you pays your money and you takes your choice but be in no doubt that a vehicle that has been well mantained with high quality lubricants will perform better, be cheaper overall to run and be far more reliable than one that is neglected or run on poor quality lubricants. This is especially true the more highly stressed (physically and thermally) particular components are (a good case in point are turbochargers which revolve at tens of thousands of RPM!)

Nowadays, so called Long Drain oils are becoming available to the every day motorist.

These Long Drain Oils contain high levels of additives and high quality base stocks in order that the oil change (drain) interval can be extended to many thousands of kilometers before the oil breaks down or becomes too contaminated with combustion by products.

They were originally developed in response to demand from trucking companies where the cost of servicing the vehicle is weighed more in the cost of having it off the road rather than the labour, lubricants and parts required.

If you take our own Long Drain 15W40 LDX Diesel Oil, it is a long drain lubricant intended for use in heavily work turbo and non-turbo truck engines with drain intervals of up to 45,000km. Hence it contains very high levels of anti-oxidants, acidic neutralisers, detergents and special viscosity improvers to ensure it can last this distance without breaking down or being overcome by excessive contamination.

However, what few folks realise is that commercial companies only extend drain intervals in conjunction with the laboratory analysis of oil samples that are taken at the normal, recommended drain interval, typically 6,000 miles. This is the only way to be sure the oil is still giving adequate protection to the engine.

Let’s take the example of an engine using a long drain oil whose drain interval has been extended to, say, 40,000km, but which has not had regular sampling and analysis of the oil within. During the 40,000km period, the vehicle could have developed faults which are not readily detectable. Examples would be: a slightly leaking head gasket or failed air filter or associated trunking; these faults would permit rapid contamination of the oil with acidic combustion by-products or highly abrasive silicon-loaded dust respectively. Without regular sampling to detect this, these contaminants would soon overcome the abilities of even a long drain oil and engine damage would result.

Given that the cost of lubricant sampling is comparable to that of a DIY oil change, then for the home mechanic extended drain intervals are not really beneficial.

Equally, if you have chosen the correct grade and specification of oil for your vehicle the additive packs within it will already exceed those required for the engine if its normal drain intervals are maintained.

Having said all of this, there are one or two engines out there which can be so heavily worked and which are subject to mechanical and thermal stresses such that the high levels of additives in a long drain oil are beneficial, even at standard service intervals. In the Land Rover stable, the only engine that fits this category is the Defender 2.5 litre turbo diesel (19J or ‘Falcon’ engine) fitted prior to the introduction of the 200 TDi.

For this engine, our Long Drain 15W40 LDX Diesel Oil will provide superb protection, so if it’s a long drain oil that you are looking for, then click HERE to buy now.