Engine Oil Comparison

Manufacturer engine oil changes... MYTH BUSTED? Shauno answers the age-old question of whether changing your oil more often REALLY makes a difference!

oil sooty

 Factory service manuals often recommend oil change intervals around 10,000kms, but Shauno explains why that might be WAY too long for your 4WD!

Watch & see what happens to the oil inside your engine if you leave it too long between oil changes, especially if you drive your car short distances without warming it up, tow or drive it hard!


Is changing your engine oil more regularly really making a difference? We test 5 000km, 10 000km and 20 000km old oil and we are going to show you the real effect of leaving your oil in your engine for too long.

You may be shocked that the service manual in your 4WD recommends that you change your oil every 10,000km. Now you think just as Shauno does, and believe that 10,000km is way too long between oil changes, especially if you use your four-wheel drive hard!


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 on a sheet of paper to take note of two things:

  1. How the oil runs down the paper, showing the viscosity and how freely it moves. The slower the oil the worse it can move around in your engine.

  2. The colour of the oil. The darker the oil means the more contaminates in the oil.

Brand new Castrol Oil

The oil is nice and clear and runs down the page smoothly.

5,000km Oil

This oil runs freely down the page nicely too, but the oil has some darkness from contaminates picked up from mixing in the components of your engine. These contaminants are things like exhaust gas, soot as well as others. The top section of this test oil stream is still actually quite clear.

Shauno changes his oil on all his 4x4s at this point, regardless of the recommended factory service manual.

10,000km Oil

It moves slowly down the paper and is noticeably darker in colour to the previous tested oils.

Now this is the recommended oil change time from most factory service recommendations in common rail diesels and even big V8 engines like the 200 Series Landcruiser.

20,000km Oil

The colour, texture and viscosity of this oil is like tar. It moves down the page as if it was glue. There is no part of this tested oil that is clear, and oil that is this contaminated could not be doing its job properly.

Hopefully this is not how long you wait to change your 4WDs oil, because this is definitely not doing your engine any favours.

engine oil km comparison

Why Engine Oil Needs Replacing?

Engine oil has two main jobs: Lubricate so there is no metal-on-metal wear, and control contamination by catching the soot created by the engine without leaving deposits that may wear down components of the engine – therefore engine oil turns dark over time.

Diesel Engines

Imagine engine oil as water, and mud as soot. If a little bit of mud makes its way into the water, it will get discoloured and dirty, but the viscosity is still pretty good. The more mud introduced (the older the oil of your engine) the thicker and darker the water will be and there are more contaminants.

The longer you leave the oil in your 4WD, the harder you work your engine by going off road, towing or in constant traffic, the more your oil will thicken and become less effective at controlling contaminations. This leaves a chance for these contaminants on components that begin to wear and rust your 4WD.

Petrol Engines

Leaving your oil in a petrol engine causes a different issue. Petrol engines don’t generate soot particles in large quantities like diesels, but the oil does get contaminated by fuel and water vapour when a petrol engine doesn’t reach optimal operating temperatures.

If a petrol engine is repeatedly driven in conditions where the oil doesn’t reach the right temperatures, nitration can occur which turns the oil brown and sludgy. This thick sludgy viscosity prevents enough oil reaching metal surfaces which can lead to premature wear.

Oil Degrading

Another reason you should change your oil before it gets too contaminated is the deposits the oil leaves in the engine are picked up by the fresh oil you put in, immediately degrading that oil.

What causes fast oil contamination?

Towing heavy loads, going four-wheel driving or any other high-stress engine work that produce more soot will lead to faster oil contamination.

Engine Operating Temperature

Short trips in your 4WD put your engine through more stress that almost all types of driving, especially in winter.

Modern diesels take more time to get to perfect operating temperatures of around 90 degrees Celsius, this takes longer to reach in slow congested traffic.

When your diesels cold the combustion process isn’t as efficient as you are going to get a lot more soot and contaminants inside your oil, in comparison to when your vehicle has had a chance to warm up on a big drive or highway kilometres.

Exhaust Gas Recirculation

Your Exhaust Gas Recirculation (EGR) will be amplified when your 4WD is cold or in traffic driving and that is not great for your oil as it is going to contaminate furthermore.

Some cases you will find that if you do a lot of stop-start driving, no large kilometres and turning off your 4WD before it reaches ideal running temperatures, your 5 000km oil will be contaminated more like 10 000km oil or worse.

Oil Comparison 2: Regular Driving vs In-Town Short Trips

We compared two test oils of 10 000km use. One was from a 4WD diesel with regular use and the other was taken from a common rail diesel strictly used in short trips to and from work in traffic.

The regular 10 000km oil acted same to the previous test – dark and slightly viscose compared to new – and the high traffic slow driven oil related more to the 20 000km old oil. This oil was very thick and very dark clearly showed this driving style contaminates your engine oil far worse.

engine oil test comparison regular vs in town traffic driving

Quality of your oil

Your driving style and the intervals in which you service your engine oil makes a huge difference in the quality and service intervals of your oil for your 4WD.

Shaun recommends choosing high quality oil like Castrol Magnatec because the better-quality oil, the better it is going to protect your engine for longer.

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.

Our 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.