Can i use synthetic oil in my car after using regular oil

Modern synthetic oil is safe to use in all types of vehicles, ranging from new purchases to classics to aging not-so-classics.
The idea that synthetic oil could harm old engines probably comes from a time before synthetic oils had been extensively tested.
Whether you use synthetic or conventional lubricant, be sure to follow the manufacturer's guidelines for oil viscosity.

So, you have a classic car and you want to put synthetic oil in it, but something you read on the internet or something your dad told you 20 years ago or just plain old anxiety is staying your hand. Never fear. We're here to clear up any confusion you have related to your car's vital fluids. We'll start off slow. Yes, you can use synthetic oil in your classic car. Or your 20-year-old Taurus. Or really anything you've got on hand. Feel better?

Table of Contents Show

Can You Mix Synthetic Oil With Regular Oil?
Mixing Motor Oil
Synthetic vs. Conventional Oil
Synthetic Oil
Conventional Oil
Possible Negative Effects of Mixing Conventional and Synthetic Oil
Can You Use Regular Oil After Synthetic?
Choose Motor Oil From Hot Shot’s Secret Today
Can you put synthetic oil after putting regular?
When should you not use synthetic oil?

If you're still skeptical, allow us to break it down. Synthetic oils are composed of chemically engineered molecules designed to provide higher performance and better protection for your engine than conventional oils, which are derived from crude oil without significant intervention from chemical engineers. Synthetic oils tend to offer better performance at extremely high or extremely low temperatures and retain their lubricating properties longer than conventional oils do.

Bob Riha Jr|Getty Images

The idea that synthetic oils are bad for older cars probably comes from the fact that early synthetic oils (we're talking 1970s) contained a chemical compound that could damage engine seals and, in some cases, cause leaks. These days, all types of motor oils are tested to make sure they won't damage your car's engine, and synthetics no longer use the chemical compound (ester) that caused these problems. If you won't take our word for it, look to the manufacturer. You should be able to find recommendations for oil in classic cars, and it's not uncommon for full synthetic to be the oil of choice. You can even look for a stamp of approval from your car's manufacturer on the back of the jug of oil, which will tell you that the company has tested that oil for compatibility with its engines.

Jeff Greenberg|Getty Images

If you're dealing with an owner's manual and recommended service intervals that were written before synthetic oil came onto the scene, you may find that you can go longer between changes after you make the switch. These oils are engineered to last longer before breaking down, so they won't start leaving sludge in your engine as soon as conventional oils would. Some synthetics have additives that are designed to help slough off sludge. If you're switching to synthetic oil for the first time, consider easing into the longer change intervals in case there are significant deposits left from bygone lubricants.

There is, of course, one downside to using synthetic oil: it's significantly more expensive than the conventional stuff. It's up to you to decide whether the performance benefits and the longer intervals between changes outweigh the cost. No matter what type of oil you decide to put in your car, make sure to follow the manufacturer's recommendation for viscosity.

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Can You Mix Synthetic Oil With Regular Oil?

The short answer is yes, you can mix synthetic oil with regular oil. However, you should only use this combination as a temporary solution until you can receive a professional oil change. While mixing motor oils is safe, doing so will limit your oil’s performance.

Mixing Motor Oil

Because most motor oils comprise a combination of additives and base oil, they’re typically compatible when mixed. However, synthetic and conventional oils have different refining processes, impacting their quality and performance.

Synthetic oil is well-refined, meaning it contains higher-quality additives and minimal impurities, keeping your engine clean and lubricated for longer. Conventional motor oil is less refined than synthetic, resulting in increased residue buildup. Mixing these oils can dilute the synthetic oil’s beneficial properties by muddying it with less refined oil.

Synthetic vs. Conventional Oil

You can better understand why mixing synthetic and conventional oils is discouraged by knowing how each motor oil is formulated.

Synthetic Oil

Synthetic motor oil comprises a petroleum-based base oil and evenly distributed high-quality powder additives. This mixture is designed to improve engine performance and protection for longer than other oils.

Due to its uniform properties, synthetic oil is highly effective across a wide temperature range, allowing for faster engine startups and longer running times between oil changes. Its low impurity levels allow for cleaner burning and slower thickening, reducing engine drag. Synthetic oil also produces fewer emissions by preventing gum and varnish buildup, promoting better fuel economy by reducing friction.

Synthetic oils are ideal for older cars that don’t receive regular oil changes, as this oil can fend off contaminants and deposits, even when the vehicle isn’t in use.

Conventional Oil

Conventional motor oil comes from natural crude oil drilled from the earth. This oil is refined through sedimentation, which removes waste products and impurities. Finally, the oil is heated and vaporized, leaving a base oil that is combined with various fluids, such as detergents, anti-oxidants, rust inhibitors and dispersants. Which liquids are added to the base oil depend on the manufacturer.

Regular oil is well-suited for newer vehicles used in light-duty applications and short-distance travel, as these tasks are less demanding for the engine.

Possible Negative Effects of Mixing Conventional and Synthetic Oil

Mixing synthetic oil with regular oil is safe but can have numerous disadvantages for your engine:

More frequent oil changes: Because conventional oil dilutes the beneficial qualities of synthetic oil, mixing them means you’ll need to get an oil change sooner.
Reduced efficiency: Conventional and synthetic oils feature different additives, chemicals and detergents that can destabilize your oil when mixed, impacting your engine’s performance.
Lower value: Combining regular oil with synthetic oil lowers the value of your synthetic oil investment, meaning you won’t get the quality or performance you paid for.

Can You Use Regular Oil After Synthetic?

Switching to conventional oil after using synthetic oil in your vehicle is safe. Because both oils compromise compatible fluids, switching to regular motor oil is okay if your car has used synthetic before. Modern technologies ensure that these oils are similar enough in their properties that no complications will occur when mixed, despite synthetic oil having higher-quality ingredients.

Choose Motor Oil From Hot Shot’s Secret Today

You can ensure optimal engine performance and longevity with oil from Hot Shot’s Secret. We’re known for creating problem-specific solutions for the automotive, trucking, RV and agricultural industries designed to increase equipment efficiency and service life. Our products are backed by science and our money-back guarantee.

Can you put synthetic oil after putting regular?

Yes. There is no danger mixing synthetic and conventional motor oil. However, conventional oil will detract from the superior performance of synthetic oil and reduce its benefits.

When should you not use synthetic oil?

This being said, however, it is still not recommended to use synthetic oil in engines that have experienced significant actions – specifically, those that have reached around 75,000 miles on the odometer.