Oils & lubricants

New Zealand has a growing number of oil companies, both independent and part of global conglomerates, and all of them are competing for your business, or more to the point, the business of your workshop customers.

So first up, how do you decide what’s the best oil to use? Do you just look at the ratings given in the handbook or on a chart, or do you look at other factors, such as average outside temperatures, or the way the vehicle is going to be used?

The Institute of Materials (IOM) is an independent organisation that conducts research on oil samples from around the world, and then provides the results to motor manufacturers, oil companies, and fleet managers.

IOM has also set down basic terms which make it easier to understand the nature of engine lubricating oil, and how they establish each classification.

The first term, and one we are probably all the most familiar with, is viscosity. This is the most important consideration when choosing an oil, since it’s what decides how fast an oil flows. The viscosity of an oil must be thick enough to lubricate and protect, but thin enough to flow through oil galleries and spread evenly over all bearing surfaces at low temperatures.

Next up is the oil grade. This is associated with viscosity in that it’s a measurement established by the Association of Automotive Engineers (SAE) to indicate how fast an oil flows. The higher the SAE number, the thicker the oil – and thick oils flow slower than thin oils.

However, oil temperature in an engine varies. At low temperatures, especially when first starting up, you need a thin oil that flows better when the engine is cold. But once the engine is running you want a thicker oil that gives better protection to the bearings.

For this reason multi-grade oils were developed, and they have two SAE grades, indicating low and high temperature characteristics.

For instance a typical SAE 10W30 oil (W stands for winter) indicates good flow characteristics when the engine is cold, while the second number refers to the oil film strength – its lubricating power – at 100 deg. C.

In colder areas, such as in the bottom of the South Island, for instance, it might be preferable to use an oil with a lower starting temperature, such as a 5W20, whereas Auckland weather wouldn’t require such a low initial “start” number.

And now here’s where it starts to get a bit complicated. Originally all oils were mineral oils, pumped out of the ground from petroleum deposits, and these oils vary in quality, in the amount of sulphur they contain, and how cleanly they burn in the engine – as happens in the combustion chamber – and in the amount of carbon deposits they leave behind on internal engine parts.

Then, led by the extra load on oils found in motor sport, aircraft engines, and space exploration, new oils were needed that would continue to protect the engine in extreme conditions. So synthetic oils were developed, with resistance to high temperature breakdown and deposit formation, coupled with additives to improve their lubricating power.

Many modern engines are designed to only use synthetic oils, especially today’s small turbo units with their tight tolerances and higher combustion temperatures.

Similarly, diesel engines, which have higher internal operating temperatures than petrol engines, require their own oils.

Synthetic oils are generally more expensive than ordinary mineral oil, but a new class of oils has been developed which combines both to give better protection than pure mineral oil, but at lower cost than pure synthetics.

Finally, there’s service classification, which is set by the American Petroleum Institute, and comprises letters which appear in the doughnut-shaped ring on oil containers.

The system rates an oil’s ability to control wear, sludge, varnish, thickening, rust, corrosion, and piston deposits.

The API service classes have two general classifications: S for “service/spark ignition” (typical passenger cars and light trucks using petrol engines), and C for “commercial/compression ignition” (typical diesel equipment). Engine oil which has been tested and meets the API standards may display the API Service Symbol (also known as the “Donut”) with the service designation on containers sold to oil users.

The latest API service standard designation is SN for petrol car and light-truck engines. The SN standard refers to a group of laboratory and engine tests, including the latest series for control of high-temperature deposits. Current API service categories include SN, SM, SL and SJ for petrol engines. All previous service designations are obsolete, although motorcycle oils commonly still use the SF/SG standard.

There are three diesel engine service designations which are current: CJ-4, CI-4, and CH-4. Some manufacturers continue to use obsolete designations such as CC for small or stationary diesel engines. In addition, API created a separated CI-4 PLUS designation in conjunction with CJ-4 and CI-4 for oils that meet certain extra requirements, and this marking is located in the lower portion of the API Service Symbol “Donut”.

It is possible for an oil to conform to both the petrol and diesel standards. In fact, it is the norm for all diesel rated engine oils to carry the “corresponding” petrol specification. For example, API CJ-4 will almost always list either SL or SM, API CI-4 with SL, API CH-4 with SJ, and so on.