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What causes dirty engine oil and is it a problem?

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Updated 10 Oct 2019

Lindsay Saunders

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The oil goes in all lovely and clear . . . then before you know it, it’s black and verging on sludge. What gives?

Engine oil is thought of as the lifeblood of your motor and there’s a good reason that cliché rings true – without it, your engine would literally stop.

It lubricates everything and it helps cool the engine too, but sadly, all that hard work doesn’t come without a cost.

There are several factors that contribute to engine oil getting dirty.

Engine surfaces

 even with the best oil in the known universe, there will be wear and tear as microscopic bits of engine chip off due to friction.

While the oil filter will catch these, it too isn’t perfect, and some tiny particles will inevitably circulate in the oil.

Combustion by-products –

 when the fuel that powers your engine burns it creates chemical by-products that are not unlike ash.

These stick to the cylinder walls and are moved through the engine by oil.

Again, the filter will get most of this, but not all.

Oil –

 speaking of chemical reactions, oil itself is changed at a molecular level with all the heating up and cooling down.

Over time that causes degradation and the creation of tiny bits and pieces that help turn your oil black.

Bypass valve –

when your engine is first started, the oil is too cold to go through the filter, so the bypass valve allows it to circulate into the engine.

When it’s doing that, contaminants in the crankcase can slip into the oil.

Once again, it’s the job of the oil filter to grab these, but that isn’t always the case, especially if your filter’s due to be replaced.

OK, so your oil’s gone black. No big deal, right? Sadly, wrong.

Dirty, sludgy oil means a less efficient engine – that is, lower performance, higher fuel consumption - as it isn’t able to do its job with all those foreign objects floating around.

It means the oil isn’t as effective at cooling the engine, increasing stress on the motor, nor keeping the hundreds of components working as smoothly as they should.

As the oil thickens, it actually makes it harder for the engine to do its job, like swimming in treacle.

It can also cause blockages and basically makes life somewhere from hard to impossible for your engine.

Having your oil and oil filters changed on time is a crucial and simple way to get the most out of any engine so stick to your mechanic’s recommended date and/or mileage for changes.

Putting it off can easily lead to big – and expensive – issues.

Luckily, AutoGuru makes it easy to book yourself in with a high-quality, local mechanic.

Search, compare and book online in just a matter of minutes!

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Written By

Lindsay Saunders

Lindsay Saunders has been writing, editing and producing words and photos for more than three decades, starting back when he drove a 1971 VW Type 3 fastback.

Now he’s got a Hyundai I30 diesel, a 1999 LWB Hi-Ace (camper project) and wishes his wife’s EJ Holden station wagon was actually his.