Synthetic oil vs. conventional oil

Are you puzzled about the types of motor oil available? Wondering if you’re choosing the right one for your car? The takeaway is that all oil is not the same. Making sure you use the correct viscosity rating is just the start. The quality and characteristics of oils can vary greatly by brand, too. To learn the difference between synthetic oil and regular oil, and everything else you need to know, you’ve come to the right place.  

What do oil weights mean?

First, before we delve into the differences of specific types of oils, it’s important to know you’re using the right oil weight for your car. 

There are two main types of oil weights, multi-viscosity and straight-weight. Multi-viscosity oils are those denoted by two numbers with a ‘W-‘ in between (for example, 10W-30 or 20W-50). The ‘W’ originally stood for winter, as oils were developed to compensate for the wear due to the vast temperature ranges then. They’re almost exclusively used today rather than straight-weight (monograde) oils. Multi-viscosity oils use polymer additives to change the natural consistency of oil as it’s heated, preventing it from thinning out as much as it otherwise would.

The only downside to this is that some of the additives used to improve viscosity at high temperatures can also leave sticky deposits. Oil companies have mostly eliminated such problems by adding detergents to oil, along with rust inhibitors and antioxidants. Some precise performance engines will still require straight-weight oil, but all newer vehicles are designed for multi-viscosity oil and require it, else your warranty will probably be void.

What the numbers mean. The numbers in motor-oil nomenclature refer to the viscosity (thickness) of the oil. A higher number corresponds to thicker oil, while a lower number refers to thinner oil. The numbers themselves are supposed to correspond to a set of real, measurable qualities in the oil, one of which is the viscosity index. In multi-viscosity oils, the left number refers to cold behavior of the particular oil, while the right number refers to its hot (100 degrees Celsius) behavior. So, for instance, 5W-30 oil would flow well when cold like 5-weight oil, but protect at high temperatures like 30-weight oil.

What is conventional motor oil?

Conventional oil is made from crude oil, the same naturally occurring oil that is used to make gasoline and other petroleum-based products. Conventional oil is less refined and therefore cheaper than synthetic motor oil.

What is synthetic motor oil?

Synthetic oil is also made from crude oil, but its manufacturing process is much more extensive, involving chemically engineered ingredients. It is more refined, which leads to higher and more consistent overall quality more suitable for extreme temperatures (both engine operating temperatures and with regards to climate), and for longer-term integrity of your car’s engine. Synthetic motor oil is available as full synthetic oils or synthetic blends.

Which is better: synthetic or conventional oil?

In the choice between synthetic vs regular oil, synthetic wins. Synthetic oil does have some drawbacks, though it’s important to weigh those against its numerous benefits.

When is synthetic oil better than conventional?

Synthetics are better in every way. They offer better high-temperature resistance and better low-temperature flow, and they leave nearly no deposits. Because synthetics flow and penetrate much better than regular oils, a change to synthetic oil will sometimes reveal leaks you didn’t know existed. Synthetic oils may also reduce emissions and maximize your car’s fuel economy.

When is conventional oil better than synthetic?

Here are the disadvantages of synthetic oil, which really boils down to one factor: price. Synthetic oils are more expensive. Prices for synthetics are coming down, though, and there are also blends that combine synthetics with traditional mineral oils.

Frequently Asked Questions

Which is better, synthetic or conventional oil?

Synthetic oil is better quality than conventional oil, and is recommended for most new vehicles.

What are the disadvantages of synthetic oil?

The only real disadvantage of synthetic oil is that it is more expensive than conventional oil.

Does conventional oil last longer than synthetic?

No. Generally, conventional oil lasts around 3,000 miles, which is less than synthetic oil.

How much longer does synthetic oil last than regular?

Synthetic oil generally lasts between 5,000 and 10,000 miles. 

Mileage for synthetic oil vs regular oil?

Rather than look for a blanket answer to the question of mileage recommendations for synthetic oil and conventional oil, it’s recommended to follow your car’s owner’s manual for guidance. Generally, synthetic oil lasts longer between oil changes, by about twice as many miles or even more, but guidelines still vary for different cars. Keep in mind, too, that your oil weight is also important. As your car ages and becomes a high-mileage vehicle, many experts recommend that you change to thicker oil than is normally used. Over time, gaps between parts in the engine become larger, enabling less oil to reach critical parts. Ask a mechanic familiar with your model vehicle or ask the dealership if you should change the type of oil for your vehicle beyond 100,000 miles.

Can you mix synthetic and conventional oil?

Yes. It won’t help, but it won’t hurt. Generally speaking, experts advise not to mix different oil brands, different oil types, or different weights. That’s because you don’t know what you’re getting. For example, if you add expensive synthetic oil to top off a car that’s running cheaper, older conventional oil, you aren’t necessarily getting the benefits of the more expensive synthetic. However, mixing oils won’t actually harm your engine.

Can you switch between regular and synthetic oil?

Yes. It’s perfectly safe to switch between regular motor oil and synthetic motor oil.

Do oils vary much by brand?

Yes. The 5W-30 oil from one company might equal 10W-40 oil from another company in viscosity, because there might be a difference in some of the other properties of the oil, like its flash point (at what temperature it ignites). The numbers in multi-viscosity oil also don’t tell anything about how viscous the oil is at normal engine operating temperatures, as opposed to extremes. The best advice here is to choose a familiar brand, and experiment with other major brands if you think it’s too thin or too thick.