Engine oils: Their distance and days are numbered

There is a significant difference between driving 5,000 km on long, steady journeys in top gear along a smooth tarmac highway with little load, and covering the same distance in short bursts at low speeds (and lower gears) in traffic, or driving in and out of a dusty quarry in low gears while carrying heavy loads of rock.

Even with a typical mix of conditions, the odometer only shows how many wheel rotations have occurred; what truly matters, and more so, is how many engine rotations there have been, along with the speed and torque during those spins.

The quality of the oil and its filter, the ambient temperature, the level of engine wear, driving habits, and even the passage of time (oil doesn’t ‘age’ much, but its additives do) all influence the rate of degradation – enough to double or halve the recommended oil change interval.

It’s no surprise that ‘recommended’ oil-change intervals can be a contentious issue! There’s simply no one-size-fits-all answer. So, even based on a broad average, ‘manufacturer’s recommendations’ are just safety-first estimates with variability.

And often, there are three different sources: the vehicle manufacturer (e.g., VW), the engine manufacturer (e.g., Cummins), and the oil manufacturer (e.g., Shell) – each with different motives and perspectives.

Ultimately, the only way to truly know the condition of the engine oil in your car is to test it. Ideally, you should test it in a laboratory to establish exactly how polluted it is, with gummy soot or more gritty particles, how functional (or not) its additives are, its viscosity, lubricity and shear strength, etc etc.

Read: Do engine oils have a shelf-life limit?

As your chances of doing that on your own car are near zero, it’s worth listening to people who have done it with sophisticated equipment on thousands of engines for decades. 

They conclude that all oils age and degrade in all those ways (albeit at different rates) and that by the time you notice any change in your engine’s performance and behaviour, significant damage will already have been done.

Their conclusion, based on a mass of statistics, is that engine oil will very rarely be adequately (never mind optimally) efficient for more than 10,000 kms (irrespective of time) or more than a year (irrespective of distance). Even brand-new oil, unused, in a sealed container, has a best-by date (of about 4 years). Read the can.

To be reasonably sure that the oil in your engine is always in good condition, the independent expert recommendation is that you change it every 5,000 kms, and if you do less mileage than that in a year, then change it annually.

Meanwhile, in a real-life context, you can test it to some degree yourself every time you do an under-bonnet check. Looking at it, feeling it, and smelling it are not scientific guarantees, but they are better than nothing.

On an older car, it might well be black (soot from engine combustion). That is neither ideal nor an automatic veto, so long as when rubbed between the fingers it is perfectly smooth (no particulate grit) and the same ‘consistency’ as new oil – slippery and strong but not gummy. 

If it feels unusually thick or thin, change it. If your engine has severely overheated, change it. And when you do, flush the old oil out while the engine is hot and fit a clean oil filter of the highest quality you can afford.