Synthetic Vs. Conventional Engine Oil Explained (And Which To Choose For Your Car)

Although most drivers might not pay it too much thought, picking the right motor oil can have a significant impact on the expected life of the engine's components. There are so many oils to pick from that working out exactly what's right for your car can be a tricky task. Price is always an important factor –- no one wants to be paying too much for an oil change -– but it's often the case that paying a premium for better oil can end up saving money in the long run.

Each car or truck manufacturer will have its own recommendations for what oil to use, and SlashGear already has buying guides for some of the most popular brands. We've covered how to pick the best oil for your Toyota and the optimal brand for your Honda, but before you start comparing products, it's best to know the difference between conventional and synthetic oil. With this guide, we'll cover all you need to know.

In the most basic sense, motor oil keeps the moving parts of an engine lubricated and prevents them from damaging each other. Conventional motor oil is made primarily out of petroleum and features additives that help it work more effectively at varying temperatures and under different driving conditions. Over time, those additives break down and stop functioning, leading to overall reduced effectiveness and potentially causing unnecessary engine wear. Since conventional oil is made up of a naturally occurring product, there is also likely to be some level of impurities in the product.

The biggest advantage of conventional motor oil is that it's significantly cheaper than all other options. Assuming your car's engine is designed to run using conventional oil and you stick to the recommended oil change intervals, it's absolutely fine to use. However, there are some key benefits that come with using synthetic oil instead.

Synthetic oil is fundamentally different from its conventional counterpart. It's artificially manufactured to have the optimal properties for the grade in question, which means it doesn't need extra additives to be effective. A crude-derived oil is still most often used as the initial base oil, but the refining and modification process of each synthetic oil manufacturer differs and is shrouded in secrecy. By altering the molecular structure of the oil, manufacturers can ensure it's free of impurities and is more resistant to breaking down than conventional oil.

The tradeoff is that all this extra manufacturing increases the price for the consumer dramatically. The best synthetic oil can be multiple times more expensive than conventional oil, although increasingly, drivers aren't getting a choice between the two. That's because an ever-increasing number of manufacturers require synthetic oil for their engines because of the multitude of advantages that it brings.

Since synthetic oil doesn't break down in the same way that conventional oil does, it generally lasts longer. Exactly how much longer will vary depending on the brand, car, and other elements such as driving style. Many top synthetic oils also feature additives that offer other benefits, such as corrosion inhibitors and detergents. Again, these will vary by brand, but they'll help with things like keeping the engine clear of sludge buildups and flushing out debris.

That, in turn, can make the engine more efficient, as there's less friction between its parts. This is one of the key reasons so many manufacturers have switched to synthetic oils as standard –- increasingly stringent emissions regulations mean that every small change counts. Increased efficiency also helps drivers spend less at the gas pump over time.

An additional benefit of synthetic oil is its increased performance in extreme temperatures. It's more effective than conventional oil at lubricating the engine on cold starts, and it'll perform better at very high temperatures as well. Turbochargers are increasingly common on modern cars, but they add additional heat that can have a detrimental effect on the life of conventional oil. Using synthetic oil offsets this problem, keeping the car running more efficiently for longer and ensuring minimal performance loss over the life of the oil.

Synthetic blends combine a mix of synthetic oil and conventional motor oil for a product that gives some of the benefits of both but comes with some compromises. It'll do a better job of withstanding temperature changes than conventional oil, but not as well as full synthetic oil. Likewise, it's a little more expensive than conventional oil, but full synthetic oil is still going to be considerably more expensive.

While synthetic blend, in theory, can be useful for those on a tighter budget, it's worth checking if your car's engine can take it. Many performance cars will require full synthetic oil, and some modern economy cars will too. Using an unsuitable blend oil in a bid to save a few dollars can be very costly in the long run, and so as a general rule of thumb, it's probably advisable to make the leap to full synthetic if you're considering switching.

Swapping from conventional to synthetic oil is a straightforward process, although advice varies slightly between brands, so it's best to check first. Mobil 1 states that, when changing to its synthetic oil engine flushing isn't necessary, while Amsoil recommends it as a precaution. Flushing the engine will remove any buildup of sludge that's left over from using conventional oil, but without flushing, the new synthetic oil will still remove those buildups over time anyway.

For higher mileage engines, Mobil 1 recommends changing the oil more frequently than normal when first switching to synthetic oil, as it will become more quickly contaminated from the sludge buildup. It may take several thousand miles for the buildups to fully flush out, and the more clogged the engine, the longer it will take. After the transition period is completed, you can then revert back to following the manufacturer's guidelines for when to change your oil.

While using the manufacturer guidelines is generally a safe bet, there are a few extra variables to consider if you want to keep your engine in peak health, especially if it has covered higher mileage. It's best to change the oil more frequently if you mostly drive short distances and don't regularly drive on the highway, as the engine won't be getting up to temperature on most of those trips. In particular, regular drives of less than five miles can cause problems.

Drivers in areas where extreme cold or hot weather is common should also change their oil more frequently, as both can cause even the best synthetic oils to break down faster than in cars operating in milder climates. Putting the engine under unusually high levels of strain – for example, heavy towing or track use – can also cause premature breakdown of oil, although exactly how frequently to change the oil will vary depending on individual usage.

It can be easy to forget about oil change intervals, especially for drivers who aren't enthusiasts and see their cars primarily as a means of getting from A to B. However, the answer to whether it's ever okay to skip an oil change is a resounding no. Even the best synthetic oil will still become less effective over time, and the consequences of driving with ineffective oil can be severe and very costly.

For newer cars, not sticking to the correct intervals can void the car's warranty, as well as cause internal damage that might shorten its lifespan. For older cars, where the engine components will already be partially worn, bad oil causes them to wear even faster or — in a worst-case scenario — even seize altogether. It's also worth noting that engine oil isn't the only oil to keep an eye on, as things like brake and transmission fluids can also potentially cause big repair bills if not properly maintained.

Once you've decided on whether to opt for conventional or synthetic oil, the next step is to pick the brand of oil to buy. Every manufacturer will have a recommended brand and that's a great place to start, but some oils offer better protection for drivers in specific situations, like high-performance use or long-distance commuting. SlashGear has handily rounded up our top picks for the best motor oil brand overall, with ten manufacturers that should cover every situation from cargo hauling to motorsport use.

If you've decided to make the switch to synthetic oil, we've also got a comparison of all the most popular brands, including our thoughts on each and how well they fare in all-important user ratings. Across both comparisons, there are a handful of brands that consistently come out on top, but even less high-performing synthetic oils will still offer significant benefits over using conventional motor oil. So, if you can justify the added cost, they're well worth the investment for most people.