There are plenty of ways to try and reduce your fuel economy; driving smoother, using your gears efficiently, never lending the car to the kids—but does the type of engine oil you buy really make a difference?

In an era where EVs are leading the charge in lowering fuel costs, could something as simple as the type of lubrication you choose be a step in the right direction for petrol-powered vehicle owners when topping up their engine oil?

And, when it comes to better understanding our cars, we’re not always as informed as we think.

In a Fixter poll, we found that 81% of motorists didn’t know how many miles their car should cover between oil changes.

The rule of thumb is between 3,000–5,000 miles, but some vehicles and oil types will perform much better, giving up to 10,000 miles between changes.

What’s the difference between synthetic and conventional engine oil?

Conventional engine oil

Conventional oil is a natural product. It began life as crude oil, mined from the ground.

Synthetic engine oil

Synthetic motor oil is a manufactured product. These oils will be refined, distilled and purified. They are broken down and rebuilt; filled with all sorts of additives, each designed to enhance engine performance.

The manufacturers fabricate the oils to reduce engine wear and sludge build-up at higher temperatures. They remove many of the original impurities of the oil and utilise each specific addition for particular engine needs.

That’s why there are so many options to choose from.

Lower oil viscosity will improve your MPG—but by how much?

Here’s the theory: the thinner the oil, the less resistance there is in your engine, and the easier it is for it to do its job.

Imagine swimming through treacle. Sounds like a lot of hard work, right? Swimming in the local pool? Much easier. Although there is a part of me that really likes the idea of the treacle option.

The oil comes in varying viscosities. Your vehicle manufacturer will have recommended which one is the most suitable for your car. Yet, when you introduce a synthetic option over a conventional oil type, the efficiency difference can be significant enough to lower your MPG.

What’s in synthetic oils that make them more efficient?

Because synthetic oil is manufactured to include performance-enhancing products, one of the key players in lowering your MPG are those designed to reduce friction.

Friction reducing additives

Manufacturers have developed methods to add some pretty clever components into their oils. One such additive, molybdenum disulphide (MoS2) can reduce friction by 65%. If you choose synthetic oil with a ‘moly’ additive, you could expect to improve fuel economy by 3 to 15%.

Thinner oils run a higher risk of evaporation

Despite engineers formulating 0W ratings (the lowest viscosity) to achieve the minimum workload on your engine’s components, it’s much more likely to evaporate at higher temperatures. It wouldn’t be able to meet the standards set by industry regulators.

Creating the perfect oil is an almost unachievable process. That’s why we need to make the best choice for our individual vehicles.

What is oil viscosity, and what do the grades mean?

The grade of oil you choose to top up the oil in your car is governed by viscosity. The grade determines the flow of oil in cold conditions and at typical high running temperatures.

So why don’t we all use the thinnest oil we can find?

The oil density determines the load the oil can carry and how much of the friction it’s designed to reduce it can carry out. Too thin, and the oil can’t do its job. Too thick, and it’s going to make your engine work harder than it needs to.

What does ‘10W-30’ and the many variations mean?

The first part of the viscosity grade, preceded by a W, is the winter viscosity. This depicts the level of flow in your engine, starting in cold winter temperatures.

The second part (the numbers after the hyphen) defines the flow at normal operating temperatures (typically 100°C).

So, a 0W-20 would be a thin, free-running oil, and at the opposite end of the spectrum, a 15W-40 would be a heavy, thick oil.

Why does the vehicle manufacturer recommend a particular oil grade?

When it comes to topping up your car oil, the recommended grade for your vehicle will take into consideration the typical use of the car, its loads, and the engine size. The manufacturer will have determined the ideal option for you to get the best out of your engine’s performance.

Depending on how the seasons perform where you live, you could choose to make accommodations for the level of heat.

It’s not only your choice of oil that can impact fuel economy—a dirty air filter will also reduce your MPG

There are so many components and consumables that affect your car’s engine performance. Did you know that even your air filter could help you achieve better fuel economy?

An over-used air filter, full of dirt and dust, clogged and unclean, prevents the correct passage of air into the fuel. That in itself can create quite an impact on your fuel economy. It’s also why you can also buy a range of different quality air filters.

Find the oil that helps you get the best from your engine, but not at the expense of its health

The next time you’re performing an engine oil check up, you’ll be more likely to consider the type of oil you choose with all of this new-found knowledge.

If you’re going to take care of business yourself, we’ve got plenty of guides outlining how to top up the oil in your car, and covering the advantages of the different oil types.

What you shouldn’t do, however, is run the risk of choosing an oil grade to improve fuel economy that won’t provide the correct level of protection for your engine. So, the next time you’re topping up the oil in your car, box clever. Saving a few pounds in petrol would be a bonus, but not if you need to replace your engine before the end of its expected lifespan.

Check your oil now—with a Fixter service