Should you change your engine oil more often?

Engine Oils Put to the Test

Is changing your engine oil more regularly really worth it? We tested 5,000km, 10,000km, and 20,000km old oil to show you the real effects of leaving oil in your engine for too long.

You might be shocked to know that many factory service manuals recommend oil changes every 10,000km. But like Shauno, you might believe that 10,000km is way too long, especially if you use your 4WD under tough conditions!

Comparing Different Km-Aged Oils

We sampled brand new Castrol 15W-40 oil, 5,000km old oil, 10,000km oil, and 20,000km oil. We poured a little bit of each sample onto a sheet of paper to observe two key factors:

1. Viscosity: How the oil flows down the paper. Slower movement indicates thicker oil, which means it may struggle to circulate properly in your engine.
2. Colour: Darker oil indicates more contaminants, such as soot and exhaust gases, which reduce the oil’s effectiveness.

Brand New Castrol Oil

• The oil is clear and flows smoothly down the page.

5,000km Oil:

• This oil still flows freely and looks relatively clean.
• However, it shows slight darkening due to contaminants like exhaust gases and soot.
• The top section of the oil stream remains clear, indicating it’s still in good condition.

Shauno changes the oil in all his 4WDs at this point, regardless of the factory service manual’s recommendations.

10,000km Oil:

• The oil flows more slowly and is noticeably darker in colour compared to the 5,000km sample.
• This is the recommended oil change interval for most factory service manuals in common rail diesels and larger engines like the 200 Series LandCruiser.

20,000km Oil:

• The oil’s colour, texture, and viscosity resemble tar.
• It flows down the page like glue, with no clear sections visible.
• Oil this contaminated cannot effectively lubricate or protect your engine.

If you’re waiting this long to change your oil, your engine is likely suffering significant wear and damage. Hopefully this is not how long you wait to change your 4WDs oil, because this is definitely not doing your engine any favours.

Left to right: Brand new oil, 5,000km, 10,000km, 20,000km.

Why Engine Oil Needs Replacing

Engine oil has two main jobs:

1. Lubrication: Prevents metal-on-metal wear inside the engine.
2. Contamination Control: Captures soot and other contaminants to prevent deposits that can wear down engine components.

Diesel Engines

Think of engine oil like water and soot like mud. A small amount of mud will discolour the water but won’t affect its viscosity much. However, as more mud (soot) is introduced, the water becomes thicker, darker, and less effective.

The longer you leave oil in a diesel engine — especially under high-stress conditions like towing, off-roading, or stop-start traffic — the more contaminants it collects. Over time, this leads to:

• Thicker, less effective oil.
• Increased wear on engine components.
• Potential rust and damage to your 4WD.

Petrol Engines

Petrol engines don’t produce as much soot as diesels, but the oil can still become contaminated by fuel and water vapour.

If a petrol engine doesn’t reach optimal operating temperatures (e.g., during short trips), nitration can occur, turning the oil brown and sludgy. This thick, sludgy oil prevents proper lubrication, leading to premature wear.

Oil Degrading

Even if you replace your oil, leaving it too long between changes can cause deposits to build up in the engine. These deposits are picked up by fresh oil, immediately degrading its quality.

What Causes Fast Oil Contamination?

• Towing Heavy Loads: Produces more soot and contaminants.
• Off-Road Driving: Puts additional stress on the engine.
• Short Trips: Prevents the engine from reaching optimal operating temperatures.
• Stop-Start Traffic: Increases soot and exhaust gas recirculation (EGR), further contaminating the oil.

Engine Operating Temperature

Short trips are one of the most stressful conditions for your engine, especially in winter.

Modern diesel engines take longer to reach their ideal operating temperature of 90°C. Until then, the combustion process is less efficient, producing more soot and contaminants.

Exhaust Gas Recirculation (EGR)

EGR systems are more active when your engine is cold or in traffic, leading to faster oil contamination.

For example, a 4WD driven only on short trips may have 5,000km oil that’s as contaminated as 10,000km oil or worse.

Oil Comparison: Regular Driving vs Short Trips

We compared two samples of 10,000km oil:

1. Regular Use: From a 4WD driven on highways and long trips.
2. Short Trips: From a 4WD primarily driven in stop-start traffic.

The regular-use oil was dark but still flowed reasonably well.

The short-trip oil, however, was thick, dark, and resembled 20,000km oil, clearly showing how short trips accelerate oil contamination.

Left to Right: Diesel 4x4, high traffic daily driver.

Quality of Your Oil

Your driving style and service intervals have a huge impact on the quality of your engine oil.

Shauno recommends using high-quality oil like Castrol Magnatec, which contains unique additives that cling to metal surfaces even during cold starts.

Most engine oils do not work their best until the engine reaches operating temperatures because the additives need heat to activate. However, Castrol Magnatec contains a unique additive found only in Castrol products that cling to metal surfaces even under cold starts. This will protect your engine while it is warming up and significantly reduce engine wear.

Recommended Oil Service Intervals

Unless your 4WD is only ever used for long highway drives and is rarely driving in traffic, we recommend shortening your oil service intervals to get the best out of your diesel motor.

Shauno recommends cutting your service intervals in half to around every 5,000kms, but newer vehicles like his 200 Series should be serviced around every 7,000kms.

The better-quality the oil, the more you can trust that the vitals of your 4WDs engine will be protected. It really is just cheap insurance for your pride and joy.