Sparesbox Blog

Engine Oil: Clearing Up All You Need to Know

By Brad Nash

Tue Jan 12 2016

With grades, cleaning power, formulations and change intervals all to consider, engine oil is far more than the simple lubricant that you pour into your engine every time you get a service or need a top up. Thankfully, all of the most common questions are more straightforward than you’d think.


1. What does the grade of my oil actually mean?

What does my engine oil grade mean?


Using the right “grade” of engine (or gearbox and differential) oil for your vehicle is absolutely crucial to ensure that your engine is properly maintained, kept clean and runs as smoothly as possible. But how does a 15W-40 differ from a 10W-40? Surely there can’t be that much of a difference, right?

Essentially, the numbers on your engine oil indicate the viscosity (if you remember year 7 science, you’ll know that viscosity refers to how freely a liquid flows without assistance) of the fluid at varying temperatures. An engine oil with a lower grade, such as a 5W-30 or a 0w-40, is more viscous and therefore flows faster than a high grade oil such as a 20W-50.

‘But why are there two numbers, and what does the “W” mean?’ I hear you ask. Basically, the “W” stands for “winter”, meaning the viscosity of the oil in cold conditions. As engine oil heats up while the motor runs, it thins out. So a 10W-40 has a grade of 10 when it’s cold, and a grade of 40 when the engine is running at normal temperature.

Engine Oils will also have a rating or specification noted on the bottle. Most commonly in the form of an ACEA rating. Your service manual will specify a particular rating as a minimum and this should always be adhered to. Always purchase an oil with the rating specified by the manufacturer.

For more details on your Engine Oil Grade, check out the latest article from our Resident Mechanic here.


2. Can I tell if my oil needs changing by looking at it?

Check Oil Dipstick
Image: Breaker Link


Not really, no. Everyone knows what used oil looks like. It’s smelly, it’s black and it generally looks the polar opposite of the pure, nourishing looking amber liquid you originally poured into your oil tank at the last service interval. Often, even as little as a quarter or halfway through your service interval, a quick check of your dip stick will reveal the same darker substance that you’d normally associate with engine oil that needs changing. However, that’s simply not the case.

Modern engine oils are formulated with detergents and additives that clean your engine’s walls and linings from all the sludge, build ups and deposits that form naturally as your engine goes about it’s daily duties. It’s these contaminants that give used engine oil the dirty consistency that you see when you finally drain it, but it’s not necessarily an indicator that it needs changing. As with the other fluids in your vehicle, always go by your vehicle manufacturer’s guidelines when determining whether to change your oil, and look for things like a change in oil pressure or temperature instead of just how your oil looks in the engine.

Speaking of changing your engine oil…


3. What’s the ultimate authority on when to change my oil?

When to Change Oil
Image: Georgetown Shell


It’s been a while since you last changed your oil, and you let it slip in a conversation to someone who thinks they know everything about cars. “Oh, every 10,000 Kilometres you HAVE to change your oil. Doesn’t matter what the manual says”, they’ll tell you.

Well, no.

The quality of modern engine oils and their capacity to clean and condition engines has changed drastically over the last couple of decades, and combined with the precision and care with which modern engines are built, there’s no reason not to go by what your vehicle manufacturer says when determining whether to change your oil. If you’re using a high quality oil at the grade recommended by your manufacturer you’ll be fine sticking with the manuals recommendation.


4. Can I use a Full-Synthetic oil in my everyday engine?

Full Synthetic Engine Oil Every Day


Fully Synthetic oil has come a long way from the early experimental formulations that originally hit the market a few decades ago. Introduced as a modern alternative to mineral oils without proper research, It was common to here stories of full-synthetic oil causing seal shrinkages and oil leaks. Now, Fully synthetic oil is standard practice from the biggest manufacturers like Nulon, Liqui-Moly, Motul, and Castrol.

As the success of full-synthetic and semi-synthetic engine oils today would indicate, this of course is no longer the case, and the enhanced cleaning power of full-synthetic engine oil means it can actually enhance the performance and longevity of your engine. It may be worth sticking to a mineral oil if you’re running an older, more worn or vintage engine, but if you have a modern car, there’s not much reason not to keep it fed with a full-synthetic oil, provided it sits within your desired budget.