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Engine oil is a vehicle's most talked-about yet least understood consumable. For most drivers, an oil change is simply an inevitable part of servicing. Yet the job oil does inside the engine goes far beyond lubrication alone, and knowing what those jobs are also explains why the change should never be put off.

What engine oil actually does

Oil performs several jobs at once inside the engine:

• Lubrication: It forms a thin film between moving metal surfaces. Thanks to this film, parts don't rub directly against one another; friction takes place within the oil layer rather than between metal surfaces.
• Cooling: Some areas of the engine, particularly the piston skirts and crankshaft bearings, are places the coolant cannot reach. The oil carries away the heat there.
• Cleaning: Soot from combustion and microscopic metal particles are picked up by the oil, held in suspension and carried to the filter. The oil darkening over time is, in most cases, a sign that it is doing this job.
• Sealing: By filling the microscopic gap between the piston rings and the cylinder wall, it reduces the escape of combustion pressure.
• Corrosion protection: It protects internal surfaces against the acidic by-products formed during combustion.

All of these jobs are supported by the additive package within the oil. The additives are depleted over time; this is the oil's chemical fatigue, and it cannot be seen with the naked eye.

What does viscosity actually tell you?

The notation you see on oil containers, such as 5W-30, describes the oil's flow behaviour. The logic behind it is as follows:

• The number at the start together with the letter W (for winter) indicates the oil's behaviour when cold. The smaller this number, the more fluid the oil stays in the cold; in other words, it reaches the upper regions of the engine more quickly on first start-up.
• The number after the dash represents the oil's viscosity at engine operating temperature. This value relates to how thick the oil film remains once hot.

The important conclusion here is this: a significant portion of an engine's wear occurs at the moment of first start-up, before the oil has yet spread everywhere. An oil with the correct fluidity when cold provides protection during these critical seconds.

It is worth being honest here: which viscosity a given vehicle should use is not a matter of preference. The engine is designed on the assumption of a specific oil film thickness and a specific flow rate. The manufacturer specifies both the viscosity and the specification/approval the oil must meet. The correct information is in your vehicle's owner's manual. Generalisations such as "a thicker oil protects more" are misleading; an oil that is thicker than required circulates too slowly through the engine's narrow, purpose-designed channels, and can produce delay rather than protection.

Why is the oil filter changed at the same time?

The filter is where the dirt collected by the oil accumulates. Changing the oil but leaving the filter means passing clean oil through a dirty reservoir on its very first circuit. A clogged filter also begins to resist flow. When this happens, the bypass valve kicks in and allows the oil to carry on without being filtered; the engine is not left without oil, but it is left without filtration. This is why the filter is a part that gets changed together with the oil.

The air filter, which is checked during the same service, also concerns the oil indirectly. A blocked air filter degrades combustion quality, and incomplete combustion increases the soot load the oil has to carry.

The consequence of delaying the change

Fatigued oil weakens quietly at first. As its additives are used up, it can no longer hold dirt in suspension, and the accumulating residue starts turning to sludge in the engine's narrow passages. As the oil film thins, friction increases; friction raises heat, and heat in turn accelerates the oil's breakdown. This is a self-feeding cycle.

The symptoms usually appear late and are subtle: the engine note hardening, a rougher idle, a slow rise in fuel consumption, the oil level dropping faster than before. Once the oil pressure warning light comes on, however, this is no longer a routine maintenance matter. The correct thing to do when this light comes on is to stop the vehicle in a safe place, switch off the engine, and have it checked.

How to check the level yourself

Checking the oil level is the easiest piece of maintenance anyone can do:

• Park the vehicle on level ground.
• Switch off the engine and wait a few minutes for the oil to settle into the sump.
• Pull out the dipstick, wipe it clean, insert it fully and pull it out again, then read the level between the marks.
• If the level is close to the lower mark, top up gradually with oil of the specification your vehicle requires, and check again.

The oil having darkened is not, on its own, a bad sign. However, a frothy, milky-coffee-like consistency may suggest water has mixed into the oil, and calls for a check without delay.

Conclusion

Engine oil is the most critical maintenance-dependent consumable in the engine, because it is the single element that lubricates, cools and cleans all at once. The correct viscosity and the correct manufacturer approval matter far more than quality claims, and this information is written in your manual. Because the change interval varies by make, engine and conditions of use, it would not be right to give a figure here; the benchmark is your vehicle's manual.

If you're having trouble keeping track of your oil change schedule, or you're not sure whether the oil you're using is suitable for your vehicle, you can book an appointment for a check.