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Changing your own oil, while simple in principle, is often a rather deceptively nuanced task in one key area: selecting which oil is best for your vehicle. For instance, there’s a huge variety of motor oil classifications covering different weights, additives, and blends. There’s also the question of whether to go synthetic or mineral (also known as conventional). Synthetic motor oil, as the name implies, comes from synthetic materials, whereas mineral-based conventional oil is refined from crude oil. But aside from the origin, is there any practical difference for your vehicle? Actually, yes — and remarkably so, depending on the type of engine you have.

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Thanks to its thinner consistency, synthetic oil tends to be better for newer engines and lasts longer than conventional oil — around 10,000 to 15,000 miles, depending on the oil you purchase and your particular vehicle. It’s also better at preventing potentially detrimental effects such as sludge buildup, and it’s the preferred oil for forced-induction engines. However, synthetic oil is usually more expensive than mineral oil, being about six to ten times more costly to manufacture.

Whether it’s worth that extra cost seems to be a matter of pure cost-benefit, at first. The more expensive synthetic oil is, generally speaking, the better option, but that’s not the full story. There are a few instances where you might want to “downgrade” to mineral oil, specifically with classic cars and other vintage applications. Let’s take a closer look at how each oil type interacts with certain engines, and which products excel where.

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Synthetic oil derives its characteristics from its refinement process. A fully synthetic oil uses no petroleum-based additives, instead deriving its manufacturing through alternative processes such as refinement from natural gas. This results in an exceptionally pure final product, but also a far more expensive one.

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Thanks to the processes involved in creating and refining synthetic oil, it features a more chemically stable and uniform molecular structure, which means synthetic oil lasts longer and produces more consistent performance than conventional oil. What’s more, synthetic oil generally lowers friction-based wear because it’s thinner and more readily operates within the tight constraints of high-performance engines. Well-refined oil also features fewer impurities and more chemical additives to increase longevity, though the precise chemicals vary depending on the make and are usually considered trade secrets.

These properties make synthetic oil the best option for modern vehicles, high-performance applications, motorsports, and turbo/supercharged engines. But that isn’t to say that a conventional oil of the same weight will ruin an engine. For around 5,000 miles or six months, conventional oil is perfectly adequate for most applications — unless your user manual states otherwise, of course. But if you plan on going further between oil changes, have a high-performance engine, or want to prevent sludge buildup and extend engine life, synthetic oil is for you. Mixing conventional and synthetic or using a synthetic blend is also perfectly acceptable, but may diminish some of the benefits of synthetic oil.

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Obviously, the main draws behind mineral-based oil are cost and availability: These oils essentially come right out of the ground and go through fewer steps before packaging, making cheap motor oil an appealing option. But this also means that you get what you pay for. Because they go through fewer refinement processes, cheap motor oils typically feature fewer additives to help bolster an engine’s reliability and performance. Moreover, these oils need to be changed more regularly, generally 2-3 times more often than synthetic. They feature more impurities and a less regular structure than synthetic oil, and are typically a thicker molecular consistency for the same weight.

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That said, conventional mineral oils offer some benefits over synthetic ones — but not that much, and certainly not in modern cars. Due to its higher viscosity and differing chemical composition, some mechanics recommend conventional oil for older vehicles and high-mileage engines (though synthetic oil is still perfectly safe to use in classic cars). In fact, many oil companies make oils specifically for use in classic cars, which are typically mineral or semi-synthetic blends.

The takeaway here is that conventional oil is perfectly adequate for some high-mileage and vintage applications, but isn’t the ideal choice for high-performance or newer vehicles. Ultimately, though, you should always defer to what it says in the owner’s manual, since that’s what the manufacturer designed the engine to accept.

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