We look at the basics of the black stuff and how it protects your car's engine...

Oil is the only barrier preventing contact between the moving metal parts inside your car's engine. It is much more than just a lubricant, though, and this viscous fluid has evolved in order to stay abreast with the technical advancements in modern internal-combustion engines. We spoke to Castrol's Rob Bowen, technology manager: Africa, to get a better understanding of engine oil in general and what you should consider when choosing a product.

Why an engine needs oil

Oil reduces friction between the moving parts, which has the effect of decreasing heat generation, lowering noise levels and, most importantly, minimising wear of the moving components. An engine's life span is directly linked to the wear rate and deterioration can occur through the following mechanisms that damage the surface material:

• Abrasion: scratching and polishing by physical contact or wear by debris and other solid contaminants in the oil, such as dirt.
• Adhesion: welding of asperities in mixed or boundary regimes.
• Corrosion: chemical reaction with the surrounding environment.
• Pitting: surface fatigue caused by rolling contact.
• Cavitation: formation and collapse of bubbles on a surface due to rapid pressure changes.

Oil's secondary roles include removing heat from hot components; providing sealing; cleaning; and keeping contaminants suspended in the liquid.

An engine's oil system

A basic lubrication system includes the following:

1. Sump or oil pan, where the oil accumulates in a wet sump application (dry sumps are for racecars, in which cornering forces may move the oil away from the oil pick-up).
2. Oil pick-up that feeds the oil pump.
3. Oil pump that supplies the oil galleries with the lubricant at pressures ranging from two to six bar depending on engine speed and application.
4. Oil filter that's tasked with removing debris from the oil.
5. Oil galleries, supplying the moving parts with oil, including the main and big-end bearings, as well as the valve train (cylinder walls are wet mostly because of spraying or splashing). The highest contact forces are on the lobes of the camshaft and can be as high as 10 tonnes per square centimetre – about the same as 10 light hatchbacks on an area the size of the sole of a stiletto's heel.
6. Oil pressure switch to inform the driver when oil pressure is lost. If all oil pressure is suddenly lost, damage occurs almost immediately.

Which oil for your engine?

It's important to stay within the manufacturer's specification for engine oil when it's time to replace it. There are, however, specially developed oils to best suit a specific application within the boundaries set by the manufacturer. These three case studies should explain it...

Case study 1 - High mileage vehicle 1997: Toyota Hilux 2,4D 
Mileage: 470 000 km
Annual mileage: 30 000 km
Usage: delivery vehicle for a small business. Daily trips vary from 5 to 100 km to carry various loads.
Proposed oil: mineral-based (Castrol GTX Diesel 15W-40).
Rob Bowen: "GTX Diesel fights diesel engine problems such as soot and harsh deposits. It's specifically selected for the African driving conditions to help clean away harmful diesel engine deposits."

Case study 2  - Performance vehicle: 2008 E92 BMW M3
Mileage: 80 000 km
Annual mileage: 10 000 km
Usage profile: a weekend vehicle for use on the open road and mountain passes. Attends one track day a year.
Proposed oil: fully synthetic, high-performance oil like Castrol Edge 5W-40.
Rob Bowen: "For a high-revving performance engine, you need an oil offering the best performance and protection to cope with the elevated engine speeds, stresses and temperatures associated with the application. Edge has additional titanium to cope with higher loads."

Case study 3 - Classic vehicle: 1968 Jaguar E-Type 4,2
120 000 km
Annual mileage: 1 000 km
Usage: driving to classic car shows and Sunday pleasure drives.
Proposed oil: a part-synthetic oil like Castrol Magnatec 10W-40.
Rob Bowen: "Although the E-Type's engine was advanced at the time, its lubrication system was not designed with the current synthetic oils in mind. Rather use a part-synthetic that offers improved additive packages (and magnetic properties in the case of Magnatec), but is closer in specification to its needs."

From crude to can

Most oil is refined from crude oil (although deriving the liquid from gas and coal is also possible) in a process called hydrocracking. Hydrogen and a catalyst are added to the heavy oil in a process that takes place in up to four reactors under high pressure and temperature. The long hydrocarbon molecules are then broken down to produce different chain lengths for various products, including base oils and fuels.

Many other processes take place at the refinery plant (for example, desulphurisation), but we’re interested in the final five groups of base oils as classified by the American Petroleum Institute (API). Groups one and two are categorised as mineral oils, and groups three to five as synthetic. The base-oil performance increases with the group number. Oil companies blend the base oils in specific ratios to create the different engine-oil ranges.

Additive packages

Each manufacturer adds additive packages during the blending process in order to meet the performance criteria of each product. These additives strive to:

• Enhance the viscosity rating;
• Lower the pour point in cold temperatures;
• Slow down oxidation at higher temperatures;
• Remove gases from the oil (anti-foaming);
• Clean the moving components (detergents);
• Keep contaminants suspended in the oil and prevent them from settling on the metal surfaces (dispersant);
• Minimise wear;
• Inhibit corrosion;
• Protect engine components under extreme pressure.

Because these additives are added during the process of creating the oil, Bowen is sceptical about any possible benefits of then adding further aftermarket additives to your car's oil: "It is like adding ingredients to a cake after it was baked."

Mineral versus synthetic

While synthetic oil might imply this product is man-made, it's slightly misleading. Yes, these oils are modified (chemically converted) and purified according to processes developed in a laboratory by the research and development engineers, but these recipes are then turned into processes to mass produce synthetic oils.

Advantages of synthetic oil over the mineral type

• Synthetic has high-quality additive packages, resulting in increased performance under extreme conditions;
• It is an cleaner oil thanks to most impurities having been removed;
• Free-flowing in cold conditions, as the wax-forming deposits are removed;
• Thermal stability is improved at high temperatures, which prevents oxidation;
• Longer service intervals are possible owing to the mentioned benefits.

The process of creating synthetic oil is more complex and costly than mineral oil and those costs are passed on to the consumer. A part- (or semi-) synthetic oil is a blend between mineral and synthetic oil. To use the term "part synthetic" on packaging material, the contents must include at least 10% synthetic oil.

Oil viscosity

Viscosity indicates resistance to flow. Water has a low viscosity, whereas syrup's viscosity is high. Oil tends to thin in higher temperatures and thicken in the cold, which negatively affects lubrication. A monograde oil (for example, SAE 40) may be effective at high temperatures, but is too thick at low temperatures.

This is why additives that modify the viscosity were developed to lower the oil's viscosity at low temperatures and increase it at high temperatures. For example, an SAE 10W-40 oil behaves like a SAE 10 monograde oil at 40 degrees Celsius, but like a SAE 40 oil at 100 degrees Celsius (the general operating point of oil in an engine).

The future of oil

Fuel economy and emissions targets are forcing the viscosity of oil lower to reduce the friction coefficient (0W-16 is now possible). Downsized turbo engines have higher internal forces and sump capacities are decreasing, and all the while drivers demand longer service intervals. Then there is the ever-present drive for a greener future, with bio-degradability becoming a key objective.

API performance level

On the back of oil bottles, you'll find an API number, for example SM. The "S" denotes a spark-ignition engine and the "M" the performance level. The performance level increases along the alphabet, and the current highest level is N.

Diesel oil will have an API such as CF, where the "C" denotes compression-ignition engines and the "F" the performance rating. "J" is the current highest diesel-oil performance rating. Many oils are developed for both petrol and diesel engines, and the API rating may be combined (example, SM/CI) to cater for both applications.

Author: Nicol Louw

 Thank you to Castrol SA for the info. Find out more at www.castrol.co.za